Adjust obsolete comment explaining set_stack_base().

Adjust obsolete comment explaining set_stack_base().

Commit 7389aad6 removed the notion of backends started from inside a
signal handler.  A stray comment still referred to them, while
explaining the need for a call to set_stack_base().  That leads to the
question of whether we still need the call in !EXEC_BACKEND builds.
There doesn't seem to be much point in suppressing it now, as it doesn't
hurt and probably helps to measure the stack base from the exact same
place in EXEC_BACKEND and !EXEC_BACKEND builds.

Back-patch to 16.

Reported-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reported-by: Tristan Partin <tristan@neon.tech>
Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKG%2BEJHcevNGNOxVWxTNFbuB%3Dvjf4U68%2B85rAC_Sxvy2zEQ%40mail.gmail.com