Add expected answers to DataFrame method examples

[comphead]

I feel it would benefit understanding for Dataframe methods if we also include the expected answer of calling the method in datafusion/core/src/dataframe/mod.rs

Currently we have a usage example and it would be great adding the expected answer

Originally posted by @comphead in #12520 (comment)

[Eason0729]

Hi, I haven't contribute here before and would like to take this issue.

[Eason0729]

I have added expected result in my fork.

Could I also change doctest data to make methods' usage more apparent?