doc: Improve pgoutput documentation.

doc: Improve pgoutput documentation.

This commit updates the pgoutput documentation with the following changes:

- Specify the data type for each pgoutput option.
- Clarify the relationship between proto_version and options such as
   streaming and two_phase.
- Add a note on the use of pg_logical_slot_peek_changes and
   pg_logical_slot_get_changes with pgoutput.

Author: Fujii Masao <[email protected]>
Reviewed-by: Chao Li <[email protected]>
Reviewed-by: Hayato Kuroda <[email protected]>
Discussion: https://postgr.es/m/CAHGQGwFJTbygdhhjR_iP4Oem=Lo1xsptWWOq825uoW+hG_Lfnw@mail.gmail.com