pgsql: docs: clarify pg_upgrade's recovery behavior

docs: clarify pg_upgrade's recovery behavior

The previous paragraph trying to explain --check, --link, and no --link
modes and the various points of failure was too complex. Instead, use
bullet lists and sublists.

Reported-by: Daniel Gustafsson

Discussion: https://postgr.es/m/qtqiv7hI87s_Xvz5ZXHCaH-1-_AZGpIDJowzlRjF3-AbCr3RhSNydM_JCuJ8DE4WZozrtxhIWmyYTbv0syKyfGB6cYMQitp9yN-NZMm-oAo=@yesql.se

Backpatch-through: 9.4

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/dfdab98cdee892ac4a557161a9aaef5715901175

Modified Files
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doc/src/sgml/ref/pgupgrade.sgml | 52 ++++++++++++++++++++++++++++-------------
1 file changed, 36 insertions(+), 16 deletions(-)