Clarify NamespaceStorageSupport.getParentNamespaceStorage()

Clarify NamespaceStorageSupport.getParentNamespaceStorage()

This override does seemingly nothing, but in fact splits invocation
groups to two -- those going to SourceSpecificContext and those
going to a NamespaceStorageSupport subclass. More specifically, if
a caller knows it is talking to NamespaceStorageSupport, JIT does
will not consider SourceSpecificContext an implementation.

Furthermore, SourceSpecificContext, as an implementation class,
guarantees it will return BuildGlobalContext.

Change-Id: Ia285f9d7745a9c114d3dd3214c2b82a651db3ce0
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>