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.