Documentation cleanup, fixing Doxygen markup. Mostly this avoids common terms

such as "protocol" and "expression" being implicitly turned into links to
mistakenly-generated Doxygen pages:
- Escaping @ symbols when Doxygen would otherwise incorrectly interpret them;
- Escaping # symbols when they're not intended as explicit Doxygen link 
  requests, such as when discussing preprocessor directives;
- In one odd case, unescaping @ in @__experimental_modules_import, because
  Doxygen wrote '\@' to the output in that case, causing the example in the
  description of ImportDecl to be wrong; and
- Fixing a typo: @breif -> @brief.

llvm-svn: 158299