pgsql: doc: Clarify wal_sender_shutdown_timeout values

doc: Clarify wal_sender_shutdown_timeout values

Clarify how special wal_sender_shutdown_timeout values affect shutdown
waiting. A value of -1 disables the shutdown timeout and lets the
WAL sender wait for the receiver to catch up, while 0 causes immediate
termination without waiting for catch-up. Positive values bound how long
shutdown waits, so document that they should be high enough for WAL data
to be replicated under normal circumstances.

Update the main documentation, the GUC long description, and
postgresql.conf.sample consistently. While here, fix nearby paragraph
indentation and spacing.

Backpatch to v19, where wal_sender_shutdown_timeout was introduced.

Author: Chao Li <lic(at)highgo(dot)com>
Reviewed-by: Ian Barwick <barwick(at)gmail(dot)com>
Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/AF4FE756-A220-4DA7-87B7-A126F3F307FD@gmail.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1aaca5ab8ffb7f9387dce2a2de128eff006ac1c4

Modified Files
--------------
doc/src/sgml/config.sgml | 32 +++++++++++++++++----------
src/backend/utils/misc/guc_parameters.dat | 2 +-
src/backend/utils/misc/postgresql.conf.sample | 4 +++-
3 files changed, 24 insertions(+), 14 deletions(-)