Other multi-word command line options use hyphens to separate
the words, so the --loglevel option introduced in CMake 3.15 was
inconsistent in this regard. Rename it to --log-level but still support
the original --loglevel name to preserve backward compatibility.
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
