Add another 3.0.0 note

Add another 3.0.0 note

Our use of Strings for YANG identifiers is confusing to the users
who need to map that string to other namespaces. Expressing the fact
that the string is composed of a limited set of characters would
be beneficial to, for example, mdsal's codegen components.

On the other hand, paying the cost of class encapsulation may not
be feasible and we should just be documenting contractual contents.

Change-Id: I3557bd180a63881b26848f087940ae62816e4214
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>