DOC: DataFrame.groupby() includes no mention of some important functionality
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- [X] I have checked that the issue still exists on the latest versions of the docs on `master` [here](https://pandas.pydata.org/docs/dev/)


### Location of the documentation

https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.groupby.html

### Documentation problem

The reference for `DataFrame.groupby` is absent of any reference to [iterating through groups](https://pandas.pydata.org/pandas-docs/dev/user_guide/groupby.html#iterating-through-groups), which seems like a fairly critical piece of information (I ended up brewing my own solution, before eventually finding the user guide link). A link to the user guide is included, but it's easy to miss.

### Suggested fix for documentation

The documentation could be improved by including some suggestions of what one might find in the user guide. For example, the text "See the user guide for more." could be replaced with something like the following:

> See the user guide for detailed usage and examples, including splitting an object into groups, iterating through groups, selecting a group, aggregation, and more.