pgsql: Update backup documentation for new APIs

Update backup documentation for new APIs

This includes the rest of the documentation that was not included
in 7117685. A larger restructure would still be wanted, but with
this commit the documentation of the new features is complete.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/cfb863f20a2a005ac89f393265d4c37ad9baab41

Modified Files
--------------
doc/src/sgml/backup.sgml | 178 ++++++++++++++++++++++++++++++++++++++---------
1 file changed, 147 insertions(+), 31 deletions(-)

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