pgsql: Move description of libpqwalreceiver hooks out of the replicatio

Move description of libpqwalreceiver hooks out of the replication's README

src/backend/replication/README includes since 32bc08b a basic
description of the WAL receiver hooks available in walreceiver.h for a
module like libpqwalreceiver, but the README has never been updated to
reflect changes done to the hooks, so the contents of the README have
rotten with the time. This commit moves the description from the README
to walreceiver.h, where it will be hard to miss that a description
update or addition is needed depending on the modifications done to the
hooks.

Each hook now includes a description of what it does in walreceiver.h,
and the replication's README mentions walreceiver.h.

Thanks also to Amit Kapila for the discussion.

Author: Michael Paquier
Reviewed-by: Peter Eisentraut
Discussion: https://postgr.es/m/20200502024606.GA471944@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/641dd167a3424f97eed8416b5ef1c21dbbf8cf12

Modified Files
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src/backend/replication/README | 29 +-------
src/include/replication/walreceiver.h | 136 ++++++++++++++++++++++++++++++++--
2 files changed, 132 insertions(+), 33 deletions(-)