Rollup merge of #141612 - jhpratt:phantom-docs, r=tgross35

Call out possibility of invariant result in variance markers

ref #135806 (comment)

Author: jhpratt

library/core/src/marker/variance.rs

@@ -131,6 +131,8 @@ phantom_lifetime! {
     ///
     /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance
     ///
+    /// Note: If `'a` is otherwise contravariant or invariant, the resulting type is invariant.
+    ///
     /// ## Layout
     ///
     /// For all `'a`, the following are guaranteed:
@@ -146,6 +148,8 @@ phantom_lifetime! {
     ///
     /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance
     ///
+    /// Note: If `'a` is otherwise covariant or invariant, the resulting type is invariant.
+    ///
     /// ## Layout
     ///
     /// For all `'a`, the following are guaranteed:
@@ -180,6 +184,8 @@ phantom_type! {
     ///
     /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance
     ///
+    /// Note: If `T` is otherwise contravariant or invariant, the resulting type is invariant.
+    ///
     /// ## Layout
     ///
     /// For all `T`, the following are guaranteed:
@@ -196,6 +202,8 @@ phantom_type! {
     ///
     /// [1]: https://doc.rust-lang.org/stable/reference/subtyping.html#variance
     ///
+    /// Note: If `T` is otherwise covariant or invariant, the resulting type is invariant.
+    ///
     /// ## Layout
     ///
     /// For all `T`, the following are guaranteed: