Make argument names of pg_get_object_address consistent, and fix docs.

Make argument names of pg_get_object_address consistent, and fix docs.

pg_get_object_address and pg_identify_object_as_address are supposed
to be inverses, but they disagreed as to the names of the arguments
representing the textual form of an object address.  Moreover, the
documented argument names didn't agree with reality at all, either
for these functions or pg_identify_object.

In HEAD and v11, I think we can get away with renaming the input
arguments of pg_get_object_address to match the outputs of
pg_identify_object_as_address.  In theory that might break queries
using named-argument notation to call pg_get_object_address, but
it seems really unlikely that anybody is doing that, or that they'd
have much trouble adjusting if they were.  In older branches, we'll
just live with the lack of consistency.

Aside from fixing the documentation of these functions to match reality,
I couldn't resist the temptation to do some copy-editing.

Per complaint from Jean-Pierre Pelletier.  Back-patch to 9.5 where these
functions were introduced.  (Before v11, this is a documentation change
only.)

Discussion: https://postgr.es/m/CANGqjDnWH8wsTY_GzDUxbt4i=y-85SJreZin4Hm8uOqv1vzRQA@mail.gmail.com