[R] Improve docs for calling Arrow compute functions directly from R

Add something about:
* where in the C++ code to find the options classes
* the fact that they have to be passed in as items on the `options` list
* the difference between unary/binary functions

Locate the Arrow Cookbook documentation for calling compute functions directly from R and trace the related C++ options classes. Document where those classes are found, how options are supplied in the options list, and how unary and binary functions differ. Done means all three points are clearly explained for R users.

Written by the indexing model from the issue text.