[AttrDocs] document always_inline

GNU documentaion for always_inline:
https://gcc.gnu.org/onlinedocs/gcc/Inline.html

GNU documentation for function attributes:
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html

Microsoft documentation for __force_inline:
https://docs.microsoft.com/en-us/cpp/cpp/inline-functions-cpp

Reviewed By: ojeda

Differential Revision: https://reviews.llvm.org/D68410