Switch from a -Z option to a cargo-features feature

This change affects Cargo.toml syntax, so use cargo-features to enable
it.

Author: joshtriplett

text/3176-cargo-multi-dep-artifacts.md

@@ -123,10 +123,10 @@ are specified in terms of the original crate name, not the dependency name;
 thus, Cargo does not currently support overriding profile settings differently
 for different artifact dependencies.
 
-Until this feature is stabilized, it will require specifying the nightly-only
-option `-Z multidep` to `cargo`. If `cargo` encounters multiple dependencies on
-the same crate and does not have this option specified, it will continue to
-emit an error.
+Until this feature is stabilized, it will require specifying the "multidep"
+feature in the nightly-only `cargo-features` key in `Cargo.toml`.  If Cargo
+encounters multiple dependencies on the same crate and does not have this
+option specified, it will continue to emit an error.
 
 # Drawbacks
 [drawbacks]: #drawbacks