Sample profiles - Add documentation for binary profile encoding. NFC.

This adds documentation for the binary profile encoding and moves the
documentation for the text encoding into the header file
SampleProfReader.h.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250309 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed