PGD: document new function "bdr.galloc_chunk_info()" (WIP)

[ibarwick]

What Changed?

This function removes the need to query galloc sequences allocated on the local node by referencing hidden rows by their CTID, which is really not something we want end users to be doing.

Add reference section, and update the section where we were telling end users to query individual tuples by CTID.

DOCS-1258.