Explain that the commands affect only the current CMake process,
not the process from which CMake was called,
nor the system environment at large,
nor the environment of subsequent build or test processes.
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
Much of the information in the old set() command documentation is now
covered in the cmake-language(7) manual. Rewrite the documentation
with this in mind. Split up the signatures for each kind of variable
into different subsections.
