pgsql: Fix failure to follow the directions when "init" fork was added.

Fix failure to follow the directions when "init" fork was added.

Specifically this commit updates forkname_to_number() so that the HINT
message includes "init" fork, and also adds the description of "init" fork
into pg_relation_size() document.

This is a part of the commit 2d00190495b22e0d0ba351b2cda9c95fb2e3d083
which has fixed the same oversight in master and 9.4. Back-patch to
9.1 where "init" fork was added.

Branch
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/a91fcd93ca038e60034efdf681c9323e4bce4385

Modified Files
--------------
doc/src/sgml/func.sgml        |    8 +++++++-
src/backend/catalog/catalog.c |    3 ++-
2 files changed, 9 insertions(+), 2 deletions(-)