[LPM] Document the new helpers to make it easy to get consistent require and preserve behavior from loop passes.

[chandlerc]

Summary

Note that I'm pretty terrible at documentation. If what I've written isn't
helping, please just suggest what would make sense here. Happy to iterate and
get something useful.

[jmolloy]

LGTM, thanks!

docs/WritingAnLLVMPass.rst

 such that outer most loop is processed last.
 
 ``LoopPass`` subclasses are allowed to update loop nest using ``LPPassManager``
 interface.  Implementing a loop pass is usually straightforward.
 ``LoopPass``\ es may overload three virtual methods to do their work.  All
 these methods should return ``true`` if they modified the program, or ``false``
 if they didn't.
 
+A ``LoopPass`` subclass which is intended to run as part of the main loop pass
+pipeline needs to preserve all of the same *function* analyses that the other
+loop passes in its pipeline require. To make that easier,
+a ``getLoopAnalysisUsage`` function is provided by ``LoopUtils.h``. It can be
+called within the subclass's ``getAnalysisUsage`` override to get consistent
+and correct behavior. Analogously, ``INITIALIZE_PASS_DEPENDENCY(LoopPass)``
+will initialize this set of function analyses.
+
 The ``doInitialization(Loop *, LPPassManager &)`` method
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 .. code-block:: c++
 
   virtual bool doInitialization(Loop *, LPPassManager &LPM);
 
 The ``doInitialization`` method is designed to do simple initialization type of