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

doc/src/sgml/ref/pg_dump.sgml

@@ -138,8 +138,11 @@ PostgreSQL documentation
        <para>
         Include large objects in the dump.  This is the default behavior
         except when <option>--schema</>, <option>--table</>, or
-        <option>--schema-only</> is specified, so the <option>-b</>
-        switch is only useful to add large objects to selective dumps.
+        <option>--schema-only</> is specified.  The <option>-b</>
+        switch is therefore only useful to add large objects to dumps
+        where a specific schema or table has been requested.  Note that
+        blobs are considered data and therefore will be included when
+        --data-only is used, but not when --schema-only is.
        </para>
       </listitem>
      </varlistentry>