pgsql: Clarify pg_dump -b documentation

Clarify pg_dump -b documentation

The documentation around the -b/--blobs option to pg_dump seemed to
imply that it might be possible to add blobs to a "schema-only" dump or
similar. Clarify that blobs are data and therefore will only be
included in dumps where data is being included, even when -b is used to
request blobs be included.

The -b option has been around since before 9.2, so back-patch to all
supported branches.

Discussion: https://postgr.es/m/[email protected]

Branch
------
REL9_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/eacdd5d7b06576c025f449b5a9eca0c11c85a5a2

Modified Files
--------------
doc/src/sgml/ref/pg_dump.sgml | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)