[libc-commits] [libc] [libc][docs] Add fcntl.h POSIX header documentation (PR #188822)

[Kit Dallege via libc-commits]

kovan wrote:

Removed the AttrDocs.td change from this PR. That change was already covered by #186590 (now merged), so no separate PR is needed. This PR now only contains the fcntl.h documentation changes.

https://github.com/llvm/llvm-project/pull/188822