pgsql: Document the new custom scan APIs.

Document the new custom scan APIs.

These APIs changed somewhat subsequent to the initial commit, and may
change further in the future, but let's document what we have today.

KaiGai Kohei and Robert Haas, reviewed by Tom Lane and Thom Brown

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/82fe8b1119e4187f3d991564274607b0b4089aca

Modified Files
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doc/src/sgml/custom-scan.sgml |  285 +++++++++++++++++++++++++++++++++++++++++
doc/src/sgml/filelist.sgml    |    1 +
doc/src/sgml/postgres.sgml    |    1 +
3 files changed, 287 insertions(+)