Document how to mark features as incomplete

This was changed in rust-lang/rust#86446 so that
incompleteness is included in the delcaration.

Author: syvb

src/feature-gates.md

@@ -27,6 +27,13 @@ For example:
 (active, non_ascii_idents, "1.0.0", Some(55467), None),
 ```
 
+Features can be marked as incomplete, and trigger the warn-by-default [`incomplete_features` lint](https://doc.rust-lang.org/rustc/lints/listing/warn-by-default.html#incomplete-features) by setting their type to `incomplete`:
+
+```rust,ignore
+/// Allows unsized rvalues at arguments and parameters.
+(incomplete, unsized_locals, "1.30.0", Some(48055), None),
+```
+
 When added, the current version should be the one for the current nightly.
 Once the feature is moved to `accepted.rs`, the version is changed to that
 nightly version.