[1.7.x] Fixed typo in docs/howto/custom-lookups.txt.

Backport of 6bc343d8749b3698dd15d1de59dfc02928bb772c from master
[1.7.x] Added an "htmlview" target to open docs after building them.

This is helpful if you build and test documentation updates in
your development env.

Copied from CPython's Doc/Makefile.

Backport of b327a614eb7d885441c6a2575e10b70ac1352aae from master
[1.7.x] Refs #23964 -- Added warning about case-insensitive, unique fields used with formsets

Backport of 4d27d72d149b714431b77f2f15bad1591a9602b7 from master
[1.7.x] Fixed #23974 -- Clarified wording of FileField.save/delete save parameter.

Thanks GreenAsJade.

Backport of 1aa41dd000e0779c8b1de1f8e7aa4a0f2f752655 from master