Mark unsafe_pin_internals as incomplete.

This thus still makes it technically possible to enable the feature, and thus
to trigger UB without `unsafe`, but this is fine since incomplete features are
known to be potentially unsound (labelled "may not be safe").

This follows from the discussion at https://github.com/rust-lang/rust/pull/93176#discussion_r799413561

compiler/rustc_feature/src/active.rs

@@ -161,6 +161,9 @@ declare_features! (
     (active, staged_api, "1.0.0", None, None),
     /// Added for testing E0705; perma-unstable.
     (active, test_2018_feature, "1.31.0", None, Some(Edition::Edition2018)),
+    /// Allows non-`unsafe` —and thus, unsound— access to `Pin` constructions.
+    /// Marked `incomplete` since perma-unstable and unsound.
+    (incomplete, unsafe_pin_internals, "1.61.0", None, None),
     /// Use for stable + negative coherence and strict coherence depending on trait's
     /// rustc_strict_coherence value.
     (active, with_negative_coherence, "1.60.0", None, None),