DOCS: convention to use mtcars.by_vs in group_by

This is a suggestion for a naming convention for variables created with group_by.

I think it would be clearer to use a convention like the following:
    \<original tbl name\>.\<by grouping variable name\> 
for naming variables created with group by. 

For example, in the documentation of group_by, mtcars grouped by cyl would be named "mtcars.by_cyl", making it easier to remember which dataset was grouped and how.

If you include this suggestion, the documentation of group_by will be slightly clearer, and maybe more people will use this naming convention for grouped dataset, making the world a slightly better place :-). Otherwise, it will be fine as well. 

There are examples of group_by in dplyr documentation, but since they are included in other functions' documentation, I think it would be more concise to leave them as they are.