Add documentation for create_lifetime_defs

compiler/rustc_ast_lowering/src/lib.rs

@@ -1523,6 +1523,10 @@ fn generate_opaque_type(
         hir::OwnerNode::Item(self.arena.alloc(opaque_ty_item))
     }
 
+    /// Given a `parent_def_id`, a list of `lifetimes_in_bounds and a `remapping` hash to be
+    /// filled, this function creates new definitions for `Param` and `Fresh` lifetimes, inserts the
+    /// new definition, adds it to the remapping with the definition of the given lifetime and
+    /// returns a list of lifetimes to be lowered afterwards.
     fn create_lifetime_defs(
         &mut self,
         parent_def_id: LocalDefId,