Doc: Remove obsolete CREATE AGGREGATE note.

The planner is in fact willing to use hash aggregation when work_mem is
not set high enough for everything to fit in memory.  This has been the
case since commit 1f39bce0, which added disk-based hash aggregation.

There are a few remaining cases in which hash aggregation is avoided as
a matter of policy when the planner surmises that spilling will be
necessary.  For example, callers of choose_hashed_setop() still
conservatively avoid hash aggregation when spilling is anticipated.
That doesn't seem like a good enough reason to mention hash aggregation
in this context.

Backpatch: 13-, where disk-based hash aggregation was introduced.

doc/src/sgml/ref/create_aggregate.sgml

@@ -386,10 +386,7 @@ SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;
       If this parameter is omitted or is zero, a default estimate is used
       based on the <replaceable>state_data_type</replaceable>.
       The planner uses this value to estimate the memory required for a
-      grouped aggregate query.  The planner will consider using hash
-      aggregation for such a query only if the hash table is estimated to fit
-      in <xref linkend="guc-work-mem"/>; therefore, large values of this
-      parameter discourage use of hash aggregation.
+      grouped aggregate query.
      </para>
     </listitem>
    </varlistentry>