Make `_.iteratee` docs more explicit
Make `_.iteratee` docs more explicit

`_.iteratee` contains a huge amount of power/magic, and based upon my [personal
experience](https://jordaneldredge.com/blog/youre-underusing-underscore/), it
is one of the most underused aspects of Underscore.

The issue of `_.iteratee`'s functionality being hard to discover has been
raised before (#2052).

Hopefully this more explicit documentation can help. Additionally it lays
groundwork for documenting `_.iteratee` as user definable if #2480 ends up
getting merged.

jashkenas/underscore,keepgoingwm/underscore,jdelbello/underscore,tnga/underscore-es,keepgoingwm/underscore,rlugojr/underscore,rlugojr/underscore,documentcloud/underscore,documentcloud/underscore,akre54/underscore,jpalomar/underscore,jdelbello/underscore,akre54/underscore,tnga/underscore-es,jashkenas/underscore,jpalomar/underscore