ovsdb-data: Verify that named-uuid string is an <id>.

ovsdb-data: Verify that named-uuid string is an <id>.

The "uuid-name" that creates symbols must be an <id> but we weren't
verifying the same constraint on the "named-uuid"s that refer to symbols,
which was a bit confusing in writing transactions by hand.  This commit
fixes the inconsistency and updates the SPECS file to clarify that a
named-uuid string has to be an <id>.