Document {Config,Status}EffectiveStatement as exceptional
These two statements have a counter-intuitive lifecycle dictated
by scalability concerns. While we could go the other way and instantiate
effective views, the information conveyed is not consulted often and
lends itself to extenal caching. Document the object model behaviour of
these two statements, providing explanation and hints to end users.
JIRA: YANGTOOLS-1271
JIRA: YANGTOOLS-1272
Change-Id: I20ac9e3aae0d0cbae37c5671649cde7347016dd8
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>