pgsql: doc: Clarify that base backup progress counts uncompressed archi

doc: Clarify that base backup progress counts uncompressed archive data

The documentation described base backup progress in terms of data
streamed, which could be mistaken for the amount of data sent over the
connection when server-side compression is enabled. Clarify that the
counters measure archive data before server-side compression.

The progress report message was also documented as counting bytes from
the current tablespace, although it reports a cumulative count for the
whole backup. Correct the description to say that the count covers all
tablespaces since the backup began.

Backpatch to 15, where both the progress report message and server-side
compression were introduced.

Suggested-by: Surya Poondla <suryapoondla4(at)gmail(dot)com>
Author: Daria Lepikhova <daria(dot)n(dot)lepikhova(at)gmail(dot)com>
Reviewed-by: Surya Poondla <suryapoondla4(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAOVWO5pNe6XprMnt5OTT+-2Sm67z5GX4DzUNArEHubF0z9-PZQ@mail.gmail.com
Backpatch-through: 15

Branch
------
REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/145901ca756013b924b15ec608dc505698604dfc

Modified Files
--------------
doc/src/sgml/monitoring.sgml | 8 +++++---
doc/src/sgml/protocol.sgml | 11 +++++++----
doc/src/sgml/ref/pg_basebackup.sgml | 4 ++--
src/include/backup/basebackup_sink.h | 8 +++++---
4 files changed, 19 insertions(+), 12 deletions(-)