[XRay][Docs] Update the XRay documentation

Summary:
Update the XRay docs to mention new subcomands to the llvm-xray tool,
and details on FDR mode logging. Also list down available libraries for
use part of the LLVM distribution.

Reviewers: rSerge, pelikan, echristo, timshen, sdardis, rengolin

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D30395

llvm-svn: 296528