Fixed typo in docs/howto/custom-lookups.txt.
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.
Fixed #23968 -- Replaced list comprehension with generators and dict comprehension
Refs #23964 -- Added warning about case-insensitive, unique fields used with formsets