Generate doc comment on benchmark runner in criterion_group macro

[mcskware]

tl;dr: One-liner to add a doc comment to the generated benchmark runner fn from the criterion_group! macro.

The rustc lint missing-docs is triggered when using the criterion_group! macro. For users who specify the lint on their rustc command line, this can generate a warning that they are unable to quiet without an inner allow attribute (e.g. using #![allow(missing-docs)] at the head of their source file). This will surpress any other missing docs in their own code, which may be unexpected behavior.

I don't see any real downsides to this change, but I'm brand new to the criterion.rs codebase. Comments, discussion, and suggestions very welcome on this proposed change. For instance, I have no idea if this change warrants an entry in the changelog. Cheers!

[mcskware]

If there are any suggestions for what the generated comment should read, I'm happy to change that. The comment won't in general be visible, of course, but will show up when a user expands the macro. The current comment simply reads "The function which runs the benchmarks."