Add to FAQ "My interval went down (e.g. from 4 to 3 days) after optimizing my parameters. Why is that?"

Reported by @brishtibheja in https://forums.ankiweb.net/t/my-cards-intervals-dont-seem-to-be-going-up-when-correct-and-some-go-down/58337/2?u=anon_0000

Q: My interval went down (e.g. from 4 to 3 days) after optimizing my FSRS parameters. Why is that?
A: If you optimize your parameters, FSRS might determine that the intervalls should be shorter than it previously predicted. The reason is that the DSR values are recalculated, if you optimize your parameters.

I'll be adding a PR in a few days.

Locate the FAQ source used by Anki's documentation; the issue does not name a file or test. Add the supplied question and answer, then verify that the FAQ renders correctly and preserves the explanation about recalculated DSR values.

Written by the indexing model from the issue text.