[XRay][docs] Define requirements on installed log handlers.

Summary:
We update the documentation to define what the requirements are for the
provided XRay log handler. This is to make it clear that the function
pointer provided must do internal synchronisation and that there are no
guarantees provided by XRay on when the function shall be invoked once
it has been installed as a log handler.

Reviewers: rSerge, rengolin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26651

llvm-svn: 287073