Docs are missing any notes about which modules are gated by features

[behnam]

At a very basic level, looking at a specific module, like r2d2, doesn't make it clear that a feature needs to be enable for it to become available, since there's no mention of the feature in any of:

• https://github.com/diesel-rs/diesel/blob/master/diesel/src/r2d2.rs
• https://docs.diesel.rs/diesel/r2d2/
• or the guides I checked from http://diesel.rs/guides/.

I think it would be great to add a note to the module's docstring, so it's visible in the API docs. What do you think?