Rotten parts of src/backend/replication/README

Hi all,

The first part of src/backend/replication/README lists all the APIs
usable for a WAL receiver, but these have aged and lost track of most
changes that happened over the years.  Four functions are listed in
the README, with incorrect signatures and many others are missing:
- walrcv_connect()
- walrcv_receive()
- walrcv_send()
- walrcv_disconnect()

I think that we should clean up that.  And as it seems to me that
nobody really remembers to update this README, I would suggest to
update the first section of the README to refer to walreceiver.h for
details about each function, then move the existing API descriptions
from the README to walreceiver.h (while fixing the incomplete
descriptions of course).  This way, if a new function is added or if
an existing function is changed, it is going to be hard to miss an
update of the function descriptions.

Any thoughts?
--
Michael