pgsql: Doc: improve protocol spec for logical replication Type messages

Doc: improve protocol spec for logical replication Type messages.

protocol.sgml documented the layout for Type messages, but completely
dropped the ball otherwise, failing to explain what they are, when
they are sent, or what they're good for. While at it, do a little
copy-editing on the description of Relation messages.

In passing, adjust the comment for apply_handle_type() to make it
clearer that we choose not to do anything when receiving a Type
message, not that we think it has no use whatsoever.

Per question from Stefen Hillman.

Discussion: https://postgr.es/m/CAPgW8pMknK5pup6=T4a_UG=Cz80Rgp=KONqJmTdHfaZb0RvnFg@mail.gmail.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/886801df4a23d0ee5e2b2119470cbf300242f644

Modified Files
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doc/src/sgml/protocol.sgml | 26 +++++++++++++++++++-------
src/backend/replication/logical/worker.c | 5 ++++-
2 files changed, 23 insertions(+), 8 deletions(-)