Thread safety analysis: Document how try-acquire is handled

[aaronpuchert]

Summary

I don't think this is obvious, since try-acquire seemingly contradicts
our usual requirements of "no conditional locking".

[aaron.ballman]

LGTM, thank you for documenting this, that would be confusing without spelling it out explicitly.

clang/docs/ThreadSafetyAnalysis.rst

 *Previously:* ``EXCLUSIVE_TRYLOCK_FUNCTION``, ``SHARED_TRYLOCK_FUNCTION``
 
 These are attributes on a function or method that tries to acquire the given
 capability, and returns a boolean value indicating success or failure.
 The first argument must be ``true`` or ``false``, to specify which return value
 indicates success, and the remaining arguments are interpreted in the same way
 as ``ACQUIRE``.  See :ref:`mutexheader`, below, for example uses.
 
+Because the analysis doesn't support conditional locking, a capability is
+treated as acquired after the first branch on the return value of a try-acquire
+function.
+
+.. code-block:: c++
+
+  Mutex mu;
+  int a GUARDED_BY(mu);
+
+  void foo() {
+    bool success = mu.TryLock();
+    a = 0;         // Warning, mu is not locked.
+    if (success) {
+      a = 0;       // Ok.
+      mu.Unlock();
+    } else {
+      a = 0;       // Warning, mu is not locked.
+    }
+  }
+
 
 ASSERT_CAPABILITY(...) and ASSERT_SHARED_CAPABILITY(...)
 --------------------------------------------------------
 
 *Previously:*  ``ASSERT_EXCLUSIVE_LOCK``, ``ASSERT_SHARED_LOCK``
 
 These are attributes on a function or method that does a run-time test to see
 whether the calling thread holds the given capability.  The function is assumed