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

[Dor1s]

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.

[Dor1s]

I'm not sure if I wrote down the information correctly, PTAL :)

[morehouse]

docs/SanitizerCoverage.rst

149

Maybe add "for linkers other than LLD". Dead stripping should work 100% for LLD now.

[Dor1s]

Clarify that only linkers other than LLD are affected.

[Dor1s]

docs/SanitizerCoverage.rst

149

Thanks, Matt! Done.

[morehouse]

LGTM

docs/SanitizerCoverage.rst

void __sanitizer_cov_8bit_counters_init(char *start, char *end) {
     // Capture this array in order to read/modify the counters.
   }
 
 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``.
 
 Users need to implement a single function to capture the PC table at startup:
 
 .. code-block:: c++