pgsql: Doc: minor improvements for logical replication protocol documen

Doc: minor improvements for logical replication protocol documentation.

Where appropriate, annotate message field data types with the backend
code's name for the data type, eg XLogRecPtr or TimestampTz.  Previously
we just said "Int64" which didn't provide as much info.

Also clarify references to object OIDs, and make use of the existing
convention to denote the value of a field that must have a fixed value.

Vignesh C, reviewed by Peter Smith and Euler Taveira.

Discussion: https://postgr.es/m/CALDaNm0+fatx57KFcBopUZWQpH_tz3WKKfm-_eiTwcXui5BnhQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a5cb4f9829fbfd68655543d2d371a18a8eb43b84

Modified Files
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doc/src/sgml/protocol.sgml | 179 ++++++++++++++++++++++++++++-----------------
1 file changed, 110 insertions(+), 69 deletions(-)