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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@352890 91177308-0d34-0410-b5e6-96231b3b80d8

docs/SanitizerCoverage.rst

@@ -144,6 +144,11 @@ PC-Table
 
 **Experimental, may change or disappear in future**
 
+**Note:** this instrumentation might be incompatible with dead code stripping
+(``-Wl,-gc-sections``) for linkers other than LLD, thus resulting in a
+significant binary size overhead. For more information, see
+`Bug 34636 <https://bugs.llvm.org/show_bug.cgi?id=34636>`_.
+
 With ``-fsanitize-coverage=pc-table`` the compiler will create a table of
 instrumented PCs. Requires either ``-fsanitize-coverage=inline-8bit-counters`` or
 ``-fsanitize-coverage=trace-pc-guard``.