[AttrDocs]: document gnu_inline function attribute

Summary: This wasn't documented https://clang.llvm.org/docs/AttributeReference.html, and briefly mentioned https://clang.llvm.org/docs/UsersManual.html#differences-between-various-standard-modes.

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: efriedma, cfe-commits, srhines

Differential Revision: https://reviews.llvm.org/D51190

llvm-svn: 340987