docs: add documentation describing API Notes

API Notes are a feature which allows annotation of headers by an
auxiliary file that contains metadata for declarations pertaining to the
associated module.  This enables adding attributes to declarations
without requiring modification of the headers, enabling finer grained
control for library headers for consumers without having to modify
external headers.

Differential Revision: https://reviews.llvm.org/D88446
Reviewed By: Richard Smith, Marcel Hlopko