Thread Safety Analysis: Update to documentation.

The attribute documentation now conforms to Aaron Ballman's renaming of the
thread safety attributes, as well as the new paper that is due to be published
in the conference on Source Code Analysis and Manipulation (SCAM 2014) later
this week.  In addition, recent changes to the analysis, such as checking
of references and negative capabilities, are now documented.

llvm-svn: 218420