DOC: DataFrameGroupBy.transform
#### Location of the documentation

https://pandas.pydata.org/docs/reference/api/pandas.core.groupby.DataFrameGroupBy.transform.html

#### Documentation problem

In my eyes, the documentation makes it very unclear how `func` (which is also mislabeled `f` in the parameter descriptions) is actually applied to each group. 

General description:
> Call function producing a like-indexed DataFrame on each group and return a DataFrame having the same indexes as the original object filled with the transformed values

Description of parameter `func`:
> Function to apply to each group. [...]

I'm not sure if I'm just confused by this or if the documentation is actually misleading, because for me this is implying that `transform` takes a function which is called for each group and thus accepts a `DataFrame` as an argument.
This however is not the case, as `transform` actually applies `func` to _each column within each group_.
Only this phrase in the 'Notes' section (and the examples) somewhat hint at this functionality, in my opinion:
> if this is a DataFrame, f must support application column-by-column in the subframe

(which I find kind of confusing as well, to be honest, as I expected that the "this" in this case would always be a `DataFrame`, because it is a method of 'groupby. _DataFrame_ GroupBy')

On the other hand, the shorthand explanation in the  'See also' about `transform` section on other pages like [GroupBy.apply](https://pandas.pydata.org/docs/reference/api/pandas.core.groupby.GroupBy.apply.html) is much more concise in my opinion:
> transform: Apply function column-by-column to the GroupBy object.

Which to me, as an unexperienced pandas user, makes it crystal-clear what the function is supposed to do, as opposed to "Call function producing a like-indexed DataFrame on each group", which is rather ominous to me. It took me a while to figure out that [GroupBy.apply](https://pandas.pydata.org/docs/reference/api/pandas.core.groupby.GroupBy.apply.html) is what I actually needed.

#### Suggested fix for documentation

Change the general description and `func` parameter description to include something along the lines of "Apply function column-by-column to the GroupBy object", which is already used as a short description as mentioned before, and fix the mislabeled parameter. And maybe the "if this is a DataFrame" phrase in the notes should be changed as well.