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
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c3b33698cf88550b017620f73b94b53029897f39

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