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 2d001904
which has fixed the same oversight in master and 9.4. Back-patch to
9.1 where "init" fork was added.