[1.6.x] Refs #23964 -- Added warning about case-insensitive, unique fields used with formsets

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

Thanks GreenAsJade.

Backport of 1aa41dd000e0779c8b1de1f8e7aa4a0f2f752655 from master