[libFuzzer] clarify the limitation of fsanitize-coverage=trace-cmp

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@254337 91177308-0d34-0410-b5e6-96231b3b80d8

docs/SanitizerCoverage.rst

@@ -275,6 +275,7 @@ they will be called by the instrumented code.
   void __sanitizer_cov_trace_switch(uint64_t Val, uint64_t *Cases);
 
 This interface is a subject to change.
+The current implementation is not thread-safe and thus can be safely used only for single-threaded targets.
 
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