pgsql: Improve documentation of the role-dropping process.

Improve documentation of the role-dropping process.

In general one may have to run both REASSIGN OWNED and DROP OWNED to get
rid of all the dependencies of a role to be dropped. This was alluded to
in the REASSIGN OWNED man page, but not really spelled out in full; and in
any case the procedure ought to be documented in a more prominent place
than that. Add a section to the "Database Roles" chapter explaining this,
and do a bit of wordsmithing in the relevant commands' man pages.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/539510582bc7bfd18c08fa5292e8061ddda68a33

Modified Files
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doc/src/sgml/ref/drop_owned.sgml | 9 ++++-
doc/src/sgml/ref/drop_role.sgml | 8 ++--
doc/src/sgml/ref/drop_user.sgml | 2 +-
doc/src/sgml/ref/reassign_owned.sgml | 23 ++++++-----
doc/src/sgml/user-manag.sgml | 70 +++++++++++++++++++++++++++++++---
5 files changed, 89 insertions(+), 23 deletions(-)