DOC: Clarify deprecation warning for iloc
Changed the wording of the deprecation warning to make clearer what exactly is deprecated.

The previous formulation was confusing (see e.g. [here](https://www.reddit.com/r/learnprogramming/comments/1c6a20o/is_pandas_iloc_being_depreciated/), even though the change from the "Deprecated since..." warning in 2.2 to "Changed in version 3.0" makes it slightly better). It is not "returning a tuple" which is deprecated but to use such a callable as an input/index.
Thanks @grossardt 