Add documentation of .env vs. unquoting

[karldw]

I added a little documentation to capture @lionel-'s comments from tidyverse/dplyr#3286. Feel free to merge if this is helpful, not worries if it's not.

I added a few examples that use magrittr pipes (%>%), but \dontrun{}-ed run them to avoid making magrittr a dependency.

[lionel-]

The formulation is not quite right: it's not .env that doesn't look through parents, but the $ operator. Also it looks like the two magrittr examples are the same.

[karldw]

Oops, thanks. Is it correct to say ".env$ will not look in a parent environment, while unquoting will."?

[lionel-]

Secondly, subsetting .env with the $ operator may be brittle because $ does not look through the parents of the environment. Using .env$name in a magrittr pipeline is an instance where this poses problem because the magrittr pipe currently evaluates its operands in a child of the current environment (this is where it defines the pronoun .).

[lionel-]

Thanks!