Doc: Remove obsolete CREATE AGGREGATE note.

 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.