Clarify the semantics of put{Configuration,Operational}Data()

This just adds documentation to how the put operations interact with
pre-existing data.

Change-Id: I01979df6a22f1a6d16c04c2cb0b97d7472d62040
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Clarify the semantics of put{Configuration,Operational}Data()"