Docs: redirect "static analysis extensions" section to the analyzer site.

The notes on the objc_method_family and ns_returns_retained-type attributes
have been moved to the Objective-C section, since both are used by ARC.
The notes on analyzer_noreturn are now only on the analyzer site.

The inadequacy of these docs was noticed months ago by Jonathan Sauer;
I'm only just now getting around to cleaning them up.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170261 91177308-0d34-0410-b5e6-96231b3b80d8