Link to description of opaque types

Author: smanilov

src/solve/opaque-types.md

@@ -1,6 +1,6 @@
 # Opaque types in the new solver
 
-The way opaque types are handled in the new solver differs from the old implementation.
+The way [opaque types](./opaque-types-type-alias-impl-trait.md) are handled in the new solver differs from the old implementation.
 This should be a self-contained explanation of the behavior in the new solver.
 
 ## opaques are alias types