[1.6.x] Fixed #23817 -- Updated docs on QuerySet evaluation

Removed inaccurate info about partial evaluation after refs #18702.
Added information on modifying sliced QuerySets; refs #22503.

Backport of 327df551e89a505c5756becee97c40198f38aff2 from master
[1.6.x] Added missing backticks to form validation docs

Backport of 38ab0f13fac428dd05dad02ef7c94da5035927ad from master