Make the standard --help-* options available from all command-line
tools. Drop options that are no longer supported:
--copyright, --help-full, --help-html, --help-man,
--help-compatcommands, --help-custom-modules
De-duplicate Help/manual/*.1.rst help options by using an
OPTIONS_HELP.txt file included from each manual.
Add "Synopsis", "Description", and "See Also" manual page section
headers. These are typical of .1 man section pages. Also de-duplicate
the See Also section links using a separate LINKS.txt file included from
each manual.
Generator expressions are supported in many places and are a distinct
concept worthy of their own manual page. The old builtin documentation
was previously represented by preprocessor macros to generate it into
each place that supports them. Factor out the duplicate content into a
dedicated cmake-generator-expressions manual page and reference it from
each original location.