Coercion warning feedback for unary distinct outside aggregate function

Coercion warning feedback for unary distinct outside aggregate function

A warning is emitted when using the standalone :func:`_.sql.distinct`
function in a :func:`_sql.select` columns list outside of an aggregate
function; this function is not intended as a replacement for the use of
:meth:`.Select.distinct`. Pull request courtesy bekapono.

Fixes: #11526
Closes: #13162
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/13162
Pull-request-sha: c6e8be7a49fecb1402dc249c84e7f982c1c34821

Change-Id: Ibbdd64a922c62a7a9ead566590ad854db4066565