[Doc] Improve doc for `<Autocomplete onCreate>` and similar props

[fzaninotto]

Problem

I had to enable choice creation in an AutocompleteInput. I followed the docs and I didn't manage to do it. I had to dig down the code.

Solution

Improve choice creation documentation

Note: I know that there is still work to do for ReferenceArrayInput & co, but Rome wasn't built in a day and this is enough for a PR.