Fix up some loose ends for CURRENT_USER as RoleSpec

Fix up some loose ends for CURRENT_USER as RoleSpec

In commit 31eae6028eca4, some documents were not updated to show the new
capability; fix that.  Also, the error message you get when CURRENT_USER
and SESSION_USER are used in a context that doesn't accept them could be
clearer about it being a problem only in those contexts; so add the
word "here".

Author: Kyotaro HORIGUCHI

His patch submission also included changes to GRANT/REVOKE, but those
seemed more controversial, so I left them out.  We can reconsider these
changes later.