pgsql: Silence compiler warning on an unused variable

Silence compiler warning on an unused variable

Also, tweak wording in comments (per Andres) and documentation (myself)
to point out that it's the database's default tablespace that can be
passed as 0, not DEFAULTTABLESPACE_OID.  Robert Haas noticed the bug in
the code, but didn't update the accompanying prose.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/0aeb5ae2041520f02cabbc7083aec46733689bce

Modified Files
--------------
doc/src/sgml/func.sgml                   |    4 ++--
src/backend/utils/adt/dbsize.c           |    7 ++++---
src/backend/utils/cache/relfilenodemap.c |    2 +-
3 files changed, 7 insertions(+), 6 deletions(-)