Expand comment on the slot recheck in drop_local_obsolete_slots().

Expand comment on the slot recheck in drop_local_obsolete_slots().

The existing comment explained that a user-created slot could reuse the
same shared memory as 'local_slot' during the window between selecting a
slot to drop and locking its database, and that we therefore recheck
before dropping.  It did not, however, spell out the fuller consequence:
because local_slot points to a reusable slot-array entry, its fields may
already describe a replacement slot, so the earlier drop decision and the
slot_database used for locking could relate to an unrelated slot/database.

Expand the comment to describe this, and note that the recheck prevents
us from dropping a user-created replacement slot while the residual risk
(such as briefly locking an unrelated database) is confined to the cycle
and is acceptable given the race is rare and non-fatal.

No functional change.

Author: Fujii Masao <masao.fujii@gmail.com>
Author: Xuneng Zhou <xunengzhou@gmail.com>
Author: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwGGyEDL3dh7uJ6qPsGvnq4QK_R8+U=12CaprnzwrwaLGA@mail.gmail.com
Discussion: https://postgr.es/m/CAHGQGwHqQ1PPVFfYKVxLfRyC-byRdwSN0NeaHj9SLYV97oO5cw@mail.gmail.com