DOC: improve doc string for .aggregate and df.transform
Since #21224, operations using ``axis=1`` in df.aggregate and df.transform now work the same as when ``axis=0``.

This PR updates the methods' doc strings to reflect that. For example, we can now pass a dict to DataFrame.agg/transform, also when ``axis=1``, and DataFrame.transform now accepts an ``axis`` parameter.

Also some related minor clarifications.
Hello @topper-123! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/generic.py`](https://github.com/topper-123/pandas/blob/dfbf8e90ae4fea5f9f1aee09fa15a7b0bd55f14d/pandas/core/generic.py) !

