Help: Document macro argument caveats in more detail

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.
cmDocumentation: Fix exit after help output format warnings

After warning that a help output format is no longer supported, fix the
return value from cmDocumentation::CheckOptions to indicate that it was
still a help option that was parsed.  Otherwise CMake moves on to treat
the help output file name as a source tree and complains that it does
not exist.
Merge topic 'fix-help-format-warning-exit' into next

e7434d4 cmDocumentation: Fix exit after help output format warnings
Merge topic 'macro-args-docs' into next

f0db2e3 Help: Document macro argument caveats in more detail