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
------
REL9_4_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/39a93fd294a0442292de70f2e42e9b1da1c4e3b6

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