Improve documentation about search_path for SECURITY DEFINER functions.

Clarify that the reason for recommending that pg_temp be put last is to
prevent temporary tables from capturing unqualified table names.  Per
discussion with Albe Laurenz.

Discussion: <A737B7A37273E048B164557ADEF4A58B5386C6E1@ntex2010i.host.magwien.gv.at>