Help: Revise docs on Scripting Commands

Revise docs for all "Scripting Commands", except four find_XXX
that use a macro suite of their own.

* Take full advantage of the improved syntax highlighting.
* Make consequential use of <..> placeholders.
* Clarify things here and there in the text.

Specific improvements to some command docs:

* "math": Correct description of novel hexadecimal capability.
* "if", "foreach", "while": Provide link to "endif" etc
* "foreach", "while": Mention "break" and "continue".
* "foreach": Simplify explanation of ``RANGE`` and ``IN`` signatures;
   advise against negative arguments or reverse ranges (compare issue #18461)
* "endif", "endfunction" etc: Explain that the argument is optional and
   maintained for compatibility only

Help/command/unset.rst

@@ -3,7 +3,7 @@ unset
 
 Unset a variable, cache variable, or environment variable.
 
-::
+.. code-block:: cmake
 
   unset(<variable> [CACHE | PARENT_SCOPE])
 
@@ -24,7 +24,7 @@ for further details.
 
 ``<variable>`` can be an environment variable such as:
 
-::
+.. code-block:: cmake
 
   unset(ENV{LD_LIBRARY_PATH})