[clang] Add pointer auth documentation to contents (PR #155763)

[Kristof Beyls via cfe-commits]

kbeyls wrote:

I think this looks good.

I'm assuming that "user documentation" goes under the heading "Using Clang as a Compiler", which this PR does.

I observe that there is also a section "Design Documents". I think that the pointer authentication documentation is much closer to "user documentation" than to "Design Documents", so I agree with the section you put it in, @ojhunt .

It's easy to move it later to another section if another section would be better.

https://github.com/llvm/llvm-project/pull/155763