Document difference between `notnil` and `strictNotNil`

[konsumlamm]

It is not clear to me what the difference between the notnil and the strictNotNil experimental features is, or rather why both of them exist. The experimental manual only says:

Its implementation is different than the notnil one: defined under strictNotNil. Keep in mind the difference in option names, be careful with distinguishing them.

What does strictNotNil do that notnil doesn't? And why is it a separate feature, instead of just being part of notnil?

[Araq]

There is no reason but the fact that strictNotNil is an independent reimplementation...

[juancarlospaco]

One has to go the way of the Dodo...