[clang][docs] Use `option` directive in User's Manual

Sphinx has the `option` directive. Most option descriptions
in `clang/docs/UsersManual.rst` used it but some didn't.
This commit changes the remaining option descriptions to use
the `option` directive. This makes a consistent view in HTML.

The `option` directive automatically creates a cross-reference target.
So labeling by `.. _opt_XXX:` is almost unnecessary. However, options
with and without `no-` (e.g. `-fno-show-column`/`-fshow-column`)
cannot be distinguish for the cross-reference. So some required
`.. _opt_XXX:` directives are kept unremoved.

Differential Revision: https://reviews.llvm.org/D138088