pgsql: Docs: create some user-facing documentation about index-only sca

Docs: create some user-facing documentation about index-only scans.

We didn't have any real user documentation about how index-only scans
work or how to design indexes to exploit them. Remedy that.
Per gripe from David Johnston.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/e6dd664d0dd362235fd0af31360ecbf29434c4c1

Modified Files
--------------
doc/src/sgml/config.sgml | 6 +-
doc/src/sgml/indexam.sgml | 18 ++--
doc/src/sgml/indices.sgml | 201 ++++++++++++++++++++++++++++++++++++++++--
doc/src/sgml/maintenance.sgml | 13 +--
doc/src/sgml/storage.sgml | 6 +-
5 files changed, 217 insertions(+), 27 deletions(-)

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