Refine comments on usage WL_POSTMASTER_DEATH vs WL_EXIT_ON_PM_DEATH

Hi, Hackers!

Current comments on the usage of WL_POSTMASTER_DEATH state that it
should be used for scenarios of finishing other than immediately i.e.
returning values and waiting for postmaster dies.
In fact, in parts of the code, it's currently used to immediately exit or
throw FATAL (in the walsender and in libpq).

So I propose change the comments on WL_POSTMASTER_DEATH stating that it
could be used for both cases: for processing and setting return values if
that's needed, and for immediate exit otherwise.

Regards,
Pavel Borisov
Supabase