Enable features when building for docs.rs

[alex]

https://docs.rs/about/builds#cross-compiling

The latest release moves everything behind features (cool!). Unfortunately, a side-effect is that the docs on docs.rs are kind of useless (sad :-()).

You can use the metadata from the link above to enable all the optional features for that build, which will make the docs useful again (yay!).

[asomers]

Oops, thanks for pointing that out. Care to review the linked PR? I'll publish a patch release after that just to get the docs rebuilt. Oh, and BTW Nix adopted fine-grained feature flags back in 0.24.0. What changed in 0.27.0 is the default setting: now no features are enabled by default.