Fix up some loose ends for CURRENT_USER as RoleSpec

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: 9d396af46357df1243aff4a9ca4f4987e4fe6024
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2015-04-30T19:57:05Z
Releases: 9.5.0
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.

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