Document customizable binary operators (#33479)

* Document customizable binary operators

I learned indirectly through comments on a PR that \otimes and \oplus and \odot can be extended as binary operators yet I cannot find this fact anywhere in the documentation.  Search the docs for `oplus` yields nothing.
Somewhere there must be a list of which characters can be extended this way and it should be documented in the manual.
This PR is an attempt to get that ball rolling!

* Add url for list and fix prefix typo

* Expand docs of infix binary operators.

Author: JeffFessler

doc/src/base/math.md

@@ -176,3 +176,29 @@ Base.widemul
 Base.Math.@evalpoly
 Base.FastMath.@fastmath
 ```
+
+## Customizable binary operators
+
+Some unicode characters can be used to define new binary operators
+that support infix notation.
+For example
+```⊗(x,y) = kron(x,y)```
+defines the `⊗` (otimes) function to be the Kronecker product,
+and one can call it as binary operator using infix syntax:
+```C = A ⊗ B```
+as well as with the usual prefix syntax
+```C = ⊗(A,B)```.
+
+Other characters that support such extensions include
+\odot `⊙`
+and
+\oplus `⊕`
+
+The complete list is in the parser code:
+https://github.com/JuliaLang/julia/blob/master/src/julia-parser.scm
+
+Those that are parsed like `*` (in terms of precedence) include
+`* / ÷ % & ⋅ ∘ × |\\| ∩ ∧ ⊗ ⊘ ⊙ ⊚ ⊛ ⊠ ⊡ ⊓ ∗ ∙ ∤ ⅋ ≀ ⊼ ⋄ ⋆ ⋇ ⋉ ⋊ ⋋ ⋌ ⋏ ⋒ ⟑ ⦸ ⦼ ⦾ ⦿ ⧶ ⧷ ⨇ ⨰ ⨱ ⨲ ⨳ ⨴ ⨵ ⨶ ⨷ ⨸ ⨻ ⨼ ⨽ ⩀ ⩃ ⩄ ⩋ ⩍ ⩎ ⩑ ⩓ ⩕ ⩘ ⩚ ⩜ ⩞ ⩟ ⩠ ⫛ ⊍ ▷ ⨝ ⟕ ⟖ ⟗`
+and those that are parsed like `+` include
+`+ - |\|| ⊕ ⊖ ⊞ ⊟ |++| ∪ ∨ ⊔ ± ∓ ∔ ∸ ≏ ⊎ ⊻ ⊽ ⋎ ⋓ ⧺ ⧻ ⨈ ⨢ ⨣ ⨤ ⨥ ⨦ ⨧ ⨨ ⨩ ⨪ ⨫ ⨬ ⨭ ⨮ ⨹ ⨺ ⩁ ⩂ ⩅ ⩊ ⩌ ⩏ ⩐ ⩒ ⩔ ⩖ ⩗ ⩛ ⩝ ⩡ ⩢ ⩣`
+There are many others that are related to arrows, comparisons, and powers.