R: gapply documentation could be clearer about what the func argument is

copied from pre-existing GH PR:

https://github.com/apache/spark/pull/28350

Spent a long time this weekend trying to figure out just what exactly key is in gapply's func. I had assumed it would be a named list, but apparently not – the examples are working because schema is applying the name and the names of the output data.frame don't matter.

As near as I can tell the description I've added is correct, namely, that key is an unnamed list.