gh-36905: `src/sage/{categories,coding,plot,quadratic_forms}/`: Fix various doctest warnings
    
We fix the following warnings
```
File "src/sage/categories/finite_complex_reflection_groups.py", line
558, in sage.categories.finite_complex_reflection_groups.FiniteComplexRe
flectionGroups.ParentMethods.milnor_fiber_poset
Warning: Consider using a block-scoped tag by inserting the line 'sage:
# optional - gap3' just before this line to avoid repeating the tag 4
times
    W = ReflectionGroup(4)                     # optional - gap3
**********************************************************************
File "src/sage/categories/finite_complex_reflection_groups.py", line
565, in sage.categories.finite_complex_reflection_groups.FiniteComplexRe
flectionGroups.ParentMethods.milnor_fiber_poset
Warning: Consider using a block-scoped tag by inserting the line 'sage:
# optional - gap3' just before this line to avoid repeating the tag 6
times
    W = ReflectionGroup([4,2,2])               # optional - gap3

File "src/sage/coding/ag_code_decoders.pyx", line 32, in
sage.coding.ag_code_decoders
Warning: Consider using a block-scoped tag by inserting the line 'sage:
# long time' just before this line to avoid repeating the tag 9 times
    channel = channels.StaticErrorRateChannel(code.ambient_space(), tau)
# long time

File "src/sage/plot/graphics.py", line 1819, in
sage.plot.graphics.Graphics.show
Warning: Variable 'G' referenced here was set only in doctest marked '#
needs sage.symbolic'; '# long time, needs sage.symbolic'
    G.show(scale='semilogy')                                          #
long time

File "src/sage/quadratic_forms/ternary_qf.py", line 924, in
sage.quadratic_forms.ternary_qf.TernaryQF.find_p_neighbor_from_vec
Warning: Variable 'Q' referenced here was set only in doctest marked '#
needs sage.libs.pari'
    Q.find_p_neighbor_from_vec(3, (0,0,1))
    [315 tests, 0.38 s]
```

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URL: #36905
Reported by: David Coudert
Reviewer(s): Matthias Köppe

Author: Release Manager

src/sage/categories/finite_complex_reflection_groups.py

@@ -555,22 +555,22 @@ def milnor_fiber_poset(self):
                 sage: sum(x**P.rank(elt) for elt in P)
                 18*x^2 + 15*x + 1
 
-                sage: W = ReflectionGroup(4)                     # optional - gap3
-                sage: P = W.milnor_fiber_poset()                 # optional - gap3
-                sage: P                                          # optional - gap3
+                sage: # optional - gap3
+                sage: W = ReflectionGroup(4)
+                sage: P = W.milnor_fiber_poset(); P
                 Finite meet-semilattice containing 41 elements
-                sage: sum(x**P.rank(elt) for elt in P)           # optional - gap3
+                sage: sum(x**P.rank(elt) for elt in P)
                 24*x^2 + 16*x + 1
 
-                sage: W = ReflectionGroup([4,2,2])               # optional - gap3
-                sage: W.is_well_generated()                      # optional - gap3
+                sage: # optional - gap3
+                sage: W = ReflectionGroup([4,2,2])
+                sage: W.is_well_generated()
                 False
-                sage: P = W.milnor_fiber_poset()                 # optional - gap3
-                sage: P                                          # optional - gap3
+                sage: P = W.milnor_fiber_poset(); P
                 Finite poset containing 47 elements
-                sage: sum(x**P.rank(elt) for elt in P)           # optional - gap3
+                sage: sum(x**P.rank(elt) for elt in P)
                 16*x^3 + 24*x^2 + 6*x + 1
-                sage: P.is_meet_semilattice()                    # optional - gap3
+                sage: P.is_meet_semilattice()
                 False
             """
             I = self.index_set()

src/sage/coding/ag_code_decoders.pyx

@@ -29,17 +29,18 @@ EXAMPLES::
 The ``decoder`` is now ready for correcting vectors received from a noisy
 channel::
 
-    sage: channel = channels.StaticErrorRateChannel(code.ambient_space(), tau)  # long time
-    sage: message_space = decoder.message_space()                   # long time
-    sage: message = message_space.random_element()                  # long time
-    sage: encoder = decoder.connected_encoder()                     # long time
-    sage: sent_codeword = encoder.encode(message)                   # long time
-    sage: received_vector = channel(sent_codeword)                  # long time
-    sage: (received_vector - sent_codeword).hamming_weight()        # long time
+    sage: # long time
+    sage: channel = channels.StaticErrorRateChannel(code.ambient_space(), tau)
+    sage: message_space = decoder.message_space()
+    sage: message = message_space.random_element()
+    sage: encoder = decoder.connected_encoder()
+    sage: sent_codeword = encoder.encode(message)
+    sage: received_vector = channel(sent_codeword)
+    sage: (received_vector - sent_codeword).hamming_weight()
     4
-    sage: decoder.decode_to_code(received_vector) == sent_codeword  # long time
+    sage: decoder.decode_to_code(received_vector) == sent_codeword
     True
-    sage: decoder.decode_to_message(received_vector) == message     # long time
+    sage: decoder.decode_to_message(received_vector) == message
     True
 
 AUTHORS:

src/sage/plot/graphics.py

@@ -1816,7 +1816,7 @@ def show(self, **kwds):
         some examples.::
 
             sage: G = list_plot([10**i for i in range(10)])                         # long time, needs sage.symbolic
-            sage: G.show(scale='semilogy')                                          # long time
+            sage: G.show(scale='semilogy')                                          # long time, needs sage.symbolic
 
         ::
 

src/sage/quadratic_forms/ternary_qf.py

@@ -921,7 +921,7 @@ def find_p_neighbor_from_vec(self, p, v, mat=False):
 
         Test that it works with (0, 0, 1)::
 
-            sage: Q.find_p_neighbor_from_vec(3, (0,0,1))
+            sage: Q.find_p_neighbor_from_vec(3, (0,0,1))                                # needs sage.libs.pari
             Ternary quadratic form with integer coefficients:
             [1 3 3]
             [-2 0 -1]