Improve attribute documentation to list which spellings are used in which syntaxes.

Summary:
Instead of listing all the spellings (including attribute namespaces) in
the section heading, only list the actual attribute names there, and
list the spellings in the supported syntaxes table.

This allows us to properly describe things like [[fallthrough]], for
which we allow a clang:: prefix in C++ but not in C, and AlwaysInline,
which has one spelling as a GNU attribute and a different spelling as a
keyword, without needing to repeat the syntax description in the
documentation text.

Sample rendering: https://pste.eu/p/T1ZV.html

Reviewers: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D51473

llvm-svn: 341097