Add some docs about the new feature to the unstable book

src/doc/unstable-book/src/language-features/re-rebalance-coherence.md

+# `re_rebalance_coherence`
+
+The tracking issue for this feature is: [#55437]
+
+[#55437]: https://github.com/rust-lang/rust/issues/55437
+
+------------------------
+
+The `re_rebalance_coherence` feature tweaks the rules which trait
+impls are allowed in crates.
+The following rule is used:
+
+Given `impl<P1..=Pn> Trait<T1..=Tn> for T0`, an impl is valid only if at
+least one of the following is true:
+- `Trait` is a local trait
+- All of
+  - At least one of the types `T0..=Tn` must be a local type. Let `Ti` be the
+    first such type.
+  - No uncovered type parameters `P1..=Pn` may appear in `T0..Ti` (excluding
+    `Ti`)
+
+
+See the [RFC](https://github.com/rust-lang/rfcs/blob/master/text/2451-re-rebalancing-coherence.md) for details.