pgsql: Revamp REPACK doc refentry page

Revamp REPACK doc refentry page

The page a bit of a mess, with notes interpersed in the list of
parameters, and two "notes" subsections that weren't in the Notes
section.

Limit the commentary under "Description" and "Parameters" to a minimum;
move the existing text to appear in the "Notes" section. Both the
"Notes on Clustering" and "Notes on Resources" subsections, which were
under "Description", are moved to be under "Notes" instead. Add a new
"Notes on Concurrent Operation" subsection there, which now carries some
text that was in "Parameters", and gets some additional text to
(hopefully) explain resource consumption more clearly -- notably, the
hard limit of 104 million tuples that can be concurrently updated/deleted
before running into the palloc size limit.

Add appropriate cross-links.

Author: Radim Marek <radim(at)boringsql(dot)com>
Author: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Author: Shihao Zhong <zhong950419(at)gmail(dot)com>
Backpatch-through: 19
Discussion: https://postgr.es/m/CAJgoLk+dodrwuwCuXERGYwgjQzSwrKd+xgitYrL32oWDt39zvA@mail.gmail.com

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/53c8cfc253768266e4345d0b870e474538002195

Modified Files
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doc/src/sgml/ref/repack.sgml | 505 +++++++++++++++++++++++--------------------
1 file changed, 270 insertions(+), 235 deletions(-)