Fix docs build for docs.rs by turning on feature only on docs.rs builds

[t4lz]

Another go at #14

In my last PR I didn't think about the fact that with that approach the crate would only build on nightly.

With the new change in this PR the crate builds successfully on stable, and the docs build successfully with the command from the docs.rs build logs (which uses nightly).

[Manishearth]

I believe the actual doccfg lines should also be gated on docsrs

[t4lz]

I believe the actual doccfg lines should also be gated on docsrs

That line is already gated:

pathdiff/src/lib.rs

Line 120 in 0b55ff6

#[cfg_attr(docsrs, doc(cfg(feature = "camino")))]

Or maybe I don't get what you mean

[Manishearth]

Ah! Yes.