cmake: Implement placeholder --help-custom-modules compatibility

The implementation of the "cmake --help-custom-modules" was removed
along with the rest of the old help generation infrastructure when
documentation was converted to reStructuredText.  In order to avoid
breaking existing project builds outright when they use a custom command
that runs a command like "cmake --help-custom-modules foo.1", generate
the requested file with placeholder content explaining that the option
is not supported anymore.
Merge topic 'compat-help-custom-modules' into next

b177294 cmake: Implement placeholder --help-custom-modules compatibility