If your crate does not compile on any of the tier-1 targets, then docs.rs cannot currently generate documentation for it. (see EDIT below).
This workaround of basically "making your crate compile on any/all of the tier-1 targets" might or might not be a good idea for your crate (depends on the crate).
But cargo-travis uploads your docs, compiled with whatever flags you'd like, to gh-pages, and you can link those from crates.io. You can even compile your docs with multiple flags, and include multiple docs versions, for different targets, or combinations of targets and target features. This might be a better option than synthetically making a crate compile in a target in which it shouldn't be used just for the sake of having docs.rs support.
EDIT: the following should work with docs.rs and is IMO better than artificially making a crate compile without features:
[package.metadata.docs.rs]
rustc-args = [ "-C target-feature=+..." ]