[clang][docs] Move flag reference text from UsersManual.rst to Options.td and CommandLineReference.html

As noted [here](https://github.com/llvm/llvm-project/issues/163349#issuecomment-3445288724), there's a bunch of command line flag reference text in the [UsersManual](https://clang.llvm.org/docs/UsersManual.html). 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](https://clang.llvm.org/docs/ClangCommandLineReference.html) 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.

Read UsersManual.rst and compare its command-line flag reference sections with Options.td and the generated CommandLineReference.html documentation. Identify the reference text that belongs in the generated flag reference, then verify that the moved documentation is complete and that the UsersManual remains coherent.

Written by the indexing model from the issue text.