DOC: improve doc string for .aggregate and .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 the new reality. For example, we can now pass a dict to DataFrame.agg/transform when ``axis=1`` also, and ``DataFrame.transform`` now has an ``axis`` parameter.

There's a minor API change, as ``Series.transform`` should have a ``axis=0`` parameter to have the same API as ``Series.aggregate``.

Also some related minor clarifications.