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
Summarize the command signatures in one block at the top of the
documentation as is typical in Unix command-line tool manuals.
Make the mode keywords links to the corresponding full signature
and documentation.
Issue: #17948
Add notes about macro arguments in the foreach, if, and list commands.
Add a section to the macro command documentation explaining in detail
how macro arguments are not variables.
