update hwasan docs

Summary:
* use more readable name
* document the hwasan attribute

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D40938

llvm-svn: 320075
GitOrigin-RevId: 79990bd606c683caa12488573e0e0f71091cd485