pgsql: Rework REPACK's documentation on clustering

Rework REPACK's documentation on clustering

Commit ac58465e0618 moved the discussion on clustering from CLUSTER's
refpage to REPACK's, but somehow introduced a mistake regarding when
exactly can the seqscan-and-sort strategy be used with respect to btrees
(spoiler: it was 100% backwards). Fix that, but also restructure and
reword the whole subsection more fully to try and make it more coherent.

Author: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Author: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Reported-by: Antonin Houska <ah(at)cybertec(dot)at>
Backpatch-through: 19
Discussion: https://postgr.es/m/69CA7636-0359-42B6-B971-16A036698187@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8db08e2522954916009dccdd88a634b1658bc522

Modified Files
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doc/src/sgml/ref/repack.sgml | 75 +++++++++++++++++++++++--------------------
src/backend/commands/repack.c | 10 +++---
2 files changed, 46 insertions(+), 39 deletions(-)