psycopg2 NOTICE fixup

psycopg2 NOTICE fixup

- don't call relatively expensive isEnabledFor(), just call _log_notices
- don't reset the list if it's empty
- fix the test to use a custom function to definitely create a notice, confirmed
that PG seems to no longer create the "implicit sequence" notices
- assert that the reset of the notices works too
- update the docs to illustrate for folks who haven't worked with logging before

Change-Id: I7291e647c177d338e0ad673f3106b4d503e4b3ea
(cherry picked from commit b0bf421f1b12eeedd77ec6c39df8e5e6cc1fcc3f)