pgsql: doc: Clarify phase descriptions of pg_stat_progress_repack.

doc: Clarify phase descriptions of pg_stat_progress_repack.

The "initializing" phase was described as expected to be very brief,
but for REPACK (CONCURRENTLY), it also covers enabling and
initializing logical decoding, either of which can take a long
time. Also, the "catch-up" phase includes waiting for the ACCESS
EXCLUSIVE lock needed to swap the files, which was not mentioned.

Adding separate progress phases for these steps is left for v20; for
v19 we only clarify the description.

Author: Sami Imseih <samimseih(dot)pg(at)gmail(dot)com>
Reviewed-by: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
Reviewed-by: Manuel Reyes Bravo <manuelreyesbravo(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAN12%2BYJOeOyPYB5JS28VE_%2Brzg%3DRbzQXfj58TGfMYwv-TX9mJA%40mail.gmail.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8a2beb5f15f71dc583c03469b61f75f884c17d1b

Modified Files
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doc/src/sgml/monitoring.sgml | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)