Update SanitizerCoverage doc regarding the issue with pc-table and gc-sections.

Update SanitizerCoverage doc regarding the issue with pc-table and gc-sections.

Summary:
There is a bug for this: https://bugs.llvm.org/show_bug.cgi?id=34636
But it would be also helpful to leave a note in the docs to prevent users from
running into issues, e.g. https://crbug.com/926588.

Reviewers: morehouse

Reviewed By: morehouse

Subscribers: cfe-commits, llvm-commits, kcc

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57474

llvm-svn: 352890