warn when postgresql distinct columns used on other dialect

[Mike Bayer]

e.g. query.distinct(<cols>)

also add doc note.

[Mike Bayer]

Implement get_unique_constraints, get_check_constraints for Oracle

Pull-request: https://github.com/zzzeek/sqlalchemy/pull/326 Pull-request: https://github.com/zzzeek/sqlalchemy/pull/342

Fixes: #4002 Change-Id: I221fe8ba305fc455a03e3a5d15f803bf8ee2e8fb

→ <<cset f8b4f7289e3b>>

[Mike Bayer]

gerrit was incorrectly targeted here, that gerrit is for #4003