Updating the documentation to include an operator! for negative capability support.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@238020 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ThreadSafetyAnalysis.rst

@@ -857,6 +857,9 @@ implementation.
 
     // Assert that is mutex is currently held for read operations.
     void AssertReaderHeld() ASSERT_SHARED_CAPABILITY(this);
+    
+    // For negative capabilities.
+    const Mutex& operator!() const { return *this; }
   };