[clang][docs] Move flag reference text from UsersManual.rst to Options.td and CommandLineReference.html #165059
Description
As noted here, there's a bunch of command line flag reference text in the UsersManual. In particular, we have complete reference for clang-cl there, with all of the flags, and all the help text, which is great. Every cl flag has help text, and that diligence deserves to be recognized. :)
However, it seems like it would be more helpful to keep the help text in the generated CommandLineReference documentation, so that it serves as a complete reference of all supported flags.
The user manual is long, and it contains a significant amount of documentation for other flags that perhaps should also live in the generated flag reference docs. In general, it could use a significant editing pass, which is why I'm filing this as an open issue for some enterprising community member. I'm happy to review the change.