[NFC][NewPM] Add clarification on analysis manager proxies

[aeubanks]

Summary

Explain why you can only get a cached analysis result, not compute one
on the fly.

[asbirlea]

lgtm.

llvm/include/llvm/IR/PassManager.h

 /// For example OuterAnalysisManagerProxy<ModuleAnalysisManager, Function> is an
 /// analysis over Functions (the "inner" unit) which provides access to a Module
 /// analysis manager.  The ModuleAnalysisManager is the "outer" manager being
 /// proxied, and Modules are the "outer" IR unit.  The inner/outer relationship
 /// is valid because each Function is contained in one Module.
 ///
 /// This proxy only exposes the const interface of the outer analysis manager,
 /// to indicate that you cannot cause an outer analysis to run from within an
-/// inner pass.  Instead, you must rely on the \c getCachedResult API.
+/// inner pass.  Instead, you must rely on the \c getCachedResult API.  This is
+/// due to keeping potential future concurrency in mind. To give an example,
+/// running a module analysis before any function passes may give a different
+/// result than running it in a function pass. Both may be valid, but it would
+/// produce non-deterministic results. GlobalsAA is a good analysis example,
+/// because the cached information has the mod/ref info for all memory for each
+/// function at the time the analysis was computed. The information is still
+/// valid after a function transformation, but it may be *different* if
+/// recomputed after that transform. GlobalsAA is never invalidated.
+
 ///
 /// This proxy doesn't manage invalidation in any way -- that is handled by the
 /// recursive return path of each layer of the pass manager.  A consequence of
 /// this is the outer analyses may be stale.  We invalidate the outer analyses
 /// only when we're done running passes over the inner IR units.
 template <typename AnalysisManagerT, typename IRUnitT, typename... ExtraArgTs>
 class OuterAnalysisManagerProxy
     : public AnalysisInfoMixin<