[XRay][docs] Define requirements on installed log handlers.

[dberris]

Summary

We update the documentation to define what the requirements are for the
provided XRay log handler. This is to make it clear that the function
pointer provided must do internal synchronisation and that there are no
guarantees provided by XRay on when the function shall be invoked once
it has been installed as a log handler.

docs/XRay.rst

- ``__xray_set_handler(void (*entry)(int32_t, XRayEntryType))``: Install your
   own logging handler for when an event is encountered. See
   ``xray/xray_interface.h`` for more details.
 - ``__xray_remove_handler()``: Removes whatever the installed handler is.
 - ``__xray_patch()``: Patch all the instrumentation points defined in the
   binary.
 - ``__xray_unpatch()``: Unpatch the instrumentation points defined in the
   binary.
 
+There are some requirements on the logging handler to be installed for the
+thread-safety of operations to be performed by the XRay runtime library:
+
+- The function should be thread-safe, as multiple threads may be invoking the
+  function at the same time. If the logging function needs to do
+  synchronisation, it must do so internally as XRay does not provide any
+  synchronisation guarantees outside from the atomicity of updates to the
+  pointer.
+- The pointer provided to ``__xray_set_handler(...)`` must be live even after
+  calls to ``__xray_remove_handler()`` and ``__xray_unpatch()`` have succeeded.
+  XRay cannot guarantee that all threads that have ever gotten a copy of the
+  pointer will not invoke the function.
+
 
 Trace Analysis Tools
 --------------------
 
 We currently have the beginnings of a trace analysis tool in LLVM, which can be
 found in the ``tools/llvm-xray`` directory. The ``llvm-xray`` tool currently
 supports the following subcommands: