Xray docs with description of Flight Data Recorder binary format.

Summary:
Adding a new restructuredText file to document the trace format produced with
an FDR mode handler and read by llvm-xray toolset.

Fixed two problems in the documentation from differential review. One bad table
and a missing link in the toc.

Original commit was e97c5836a77db803fe53319c53f3bf8e8b26d2b7.

Reviewers: dberris, pelikan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36041

llvm-svn: 309891