[Docs] Update MSan docs

Summary:
1. Move MSan-specific flags and features from user manual to MSan page.
2. Update current status / list of supported platforms.

Reviewers: eugenis, kcc

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D15246

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@254788 91177308-0d34-0410-b5e6-96231b3b80d8