clarify default configuration behavior
clarify default configuration behavior

alembic does not by default read other sections
besides ``[alembic]``, however the generated ``env.py``
file by default contains instructions to also read logging
directives.  clarify that if the configuration file is
customized to not have logging directives, env.py also
needs to be customized.

Change-Id: I4804ce54b78b68c2d72ba4bcc7c1ecaf93a09103
Fixes: #985

zzzeek/alembic,sqlalchemy/alembic