[views.span] Editorial review fixes:

  * [span.sub] Refactor the specification of subspan() slightly to define the return type precisely. Note that when Extent is not dynamic, then Extent and size() are the same, so we can say "size()" in both cases unconditionally.

  * Introduce new heading "Overview [span.overview]" to remove hanging paragraphs in [views.span].

  * Rephrase "Extent < dynamic_extent" to "Extent is negative and not equal to dynamic_extent" to avoid unnecessary dependency on details of dynamic_extent.

Author: tkoeppe

source/containers.tex

@@ -9916,6 +9916,8 @@
 
 \rSec2[views.span]{Class template \tcode{span}}
 
+\rSec3[span.overview]{Overview}
+
 \pnum
 \indexlibrary{\idxcode{span}}%
 A \tcode{span} is a view over a contiguous sequence of objects,
@@ -9925,7 +9927,8 @@
 \tcode{ElementType} is required to be a complete object type that is not an abstract class type.
 
 \pnum
-If \tcode{Extent < dynamic_extent}, the program is ill-formed.
+If \tcode{Extent} is negative and not equal to \tcode{dynamic_extent},
+the program is ill-formed.
 
 \pnum
 The iterator type for span is a random access iterator and a contiguous iterator.
@@ -10246,14 +10249,17 @@
 \effects
 Equivalent to:
 \begin{codeblock}
-return span<ElementType,
-            Count != dynamic_extent
-              ? Count
-              : (Extent != dynamic_extent ? Extent - Offset : dynamic_extent)>(
-              data() + Offset,
-              Count != dynamic_extent
-                ? Count
-                : (Extent != dynamic_extent ? Extent - Offset : size() - Offset));
+return span<ElementType, @\seebelow@>(
+  data() + Offset, Count != dynamic_extent ? Count : size() - Offset);
+\end{codeblock}
+
+\pnum
+\remarks
+The second template argument of the returned \tcode{span} type is:
+\begin{codeblock}
+Count != dynamic_extent ? Count
+                        : (Extent != dynamic_extent ? Extent - Offset
+                                                    : dynamic_extent)
 \end{codeblock}
 \end{itemdescr}