pgsql: Doc: write some for adminpack.

Doc: write some for adminpack.

Previous contents of adminpack.sgml were rather far short of project norms.
Not to mention being outright wrong about the signature of pg_file_read().

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/a470858078e129641de24f5d090632f76b055cd8

Modified Files
--------------
doc/src/sgml/adminpack.sgml | 158 +++++++++++++++++++++++++++++++++++++++-----
1 file changed, 140 insertions(+), 18 deletions(-)

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