Reword memory terminology for PQresultMemorySize

Reword memory terminology for PQresultMemorySize

Rather than using the generic word "space" we might as well use "memory"
since that's precisely what we're dealing with here.

This was extracted from a larger patch around terminology changes where
the remaining hunks were rejected.

Author: Gurjeet Singh <gurjeet@singh.im>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CABwTF4UHO_NtcsOL6_XZfnpKg_0XBFKa7B-7_x5zs3MRZm3-Tg@mail.gmail.com