Doc: Clarify DROP SUBSCRIPTION behavior after SET (slot_name = NONE).

Doc: Clarify DROP SUBSCRIPTION behavior after SET (slot_name = NONE).

The previous text claimed that once the slot is disassociated with
ALTER SUBSCRIPTION ... SET (slot_name = NONE), DROP SUBSCRIPTION "will no
longer attempt any actions on a remote host".  That is inaccurate:
DROP SUBSCRIPTION may still connect to the publisher to drop
internally-created table synchronization slots when some table
synchronization is left unfinished.  Reword to describe this, and note
that if the publisher is unreachable those slots (and the main slot, if
it still exists) must be dropped manually to avoid indefinitely reserving
WAL.

Reported-by: Jeff Davis <pgsql@j-davis.com>
Author: Amit Kapila <amit.kapila16@gmail.com>
Backpatch-through: 14
Discussion: https://postgr.es/m/CAA4eK1+tyYSpPxMBy1974kjivuGeR7YY=yopwRGrK3+vCTysdg@mail.gmail.com
Discussion: https://postgr.es/m/D908370F-2695-4231-851D-17179A6A6F2A@gmail.com