NFC: Document MSVC getters on Triple

Summary:
Document the differences between the isKnownWindowsMSVC() and isWindowsMSVC() methods on Triple.

Also removed the '\brief' Doxygen annotations - now that 'AUTOBRIEF' is set to on, they are unnecessary.

Subscribers: jfb, tberghammer, danalbert, srhines, dschuff

Differential Revision: http://reviews.llvm.org/D14110

llvm-svn: 253159