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/20161119173316.GA13284@tamriel.snowman.net

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/d722927e1f0ce13a289c3dd8106265d27ca25c3b

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