[1.6.x] Fixed #21643 -- repeated execution of qs with F() + timedelta

Thanks Tim Graham for review and Tai Lee for the additional test to prove
this was a regression in 1.6.

Backport of 7f2485b4d1 and 8137215973 from master
[1.6.x] Fixed some typos in the documentation.

Thanks to Rodolfo Carvalho and Piotr Kasprzyk
for the patch.

Partial backport of ea4da8e63c314b4f7cbcb4dd06eef466c1e872ed from master.
[1.6.x] Fixed #21908 -- Added example usage for ModelAdmin.get_inline_instances().

Thanks matt at schinckel.net for the suggestion.

Backport of 5a4f1298cf from master
[1.6.x] Fixed #21986 -- Added some guidelines for database indexes.

Thanks django-issues at colons.co for the suggestion.

Backport of c8d61fa109 from master