doc: vacuum full, fillfactor, and "extra space"

I started writing this patch to avoid the possibly-misleading phrase: "with no
extra space" (since it's expected to typically take ~2x space, or 1x "extra"
space).

But the original phrase "with no extra space" seems to be wrong anyway, since
it actually follows fillfactor, so say that.  Possibly should be backpatched.

diff --git a/doc/src/sgml/ref/vacuum.sgml b/doc/src/sgml/ref/vacuum.sgml
index ec2503d..9757352 100644
--- a/doc/src/sgml/ref/vacuum.sgml
+++ b/doc/src/sgml/ref/vacuum.sgml
@@ -75,10 +75,16 @@ VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ ANALYZE ] [ <replaceable class="paramet
    with normal reading and writing of the table, as an exclusive lock
    is not obtained.  However, extra space is not returned to the operating
    system (in most cases); it's just kept available for re-use within the
-   same table.  <command>VACUUM FULL</command> rewrites the entire contents
-   of the table into a new disk file with no extra space, allowing unused
-   space to be returned to the operating system.  This form is much slower and
-   requires an exclusive lock on each table while it is being processed.
+   same table.
+  </para>
+
+  <para>
+   <command>VACUUM FULL</command> rewrites the entire contents of the table
+   into a new file on disk with internal space left available as determined by
+   <literal>fillfactor</literal>.  If the table includes many dead tuples from
+   updates/deletes, this allows unused space to be returned to the operating
+   system.  This form is much slower and requires an exclusive lock on each
+   table while it is being processed.
   </para>
 
   <para>
-- 
2.7.4