pgsql: 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.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/9d396af46357df1243aff4a9ca4f4987e4fe6024

Modified Files
--------------
doc/src/sgml/ref/alter_policy.sgml | 2 +-
doc/src/sgml/ref/create_policy.sgml | 2 +-
doc/src/sgml/ref/create_tablespace.sgml | 2 +-
doc/src/sgml/ref/drop_owned.sgml | 2 +-
doc/src/sgml/ref/reassign_owned.sgml | 3 ++-
src/backend/parser/gram.y | 4 ++--
src/test/regress/expected/rolenames.out | 4 ++--
7 files changed, 10 insertions(+), 9 deletions(-)