Refine import implied rules to smooth migration

Author: odersky

docs/docs/reference/contextual/import-implied.md

@@ -27,3 +27,25 @@ There are two main benefits arising from these rules:
    without importing anything else. This is particularly important since implied
    instances can be anonymous, so the usual recourse of using named imports is not
    practical.
+
+### Relationship with Old-Style Implicits
+
+The rules of implied imports above have the consequence that a library
+would have to migrate in lockstep with all its users from old style implicits and
+normal imports to implied instances and imports.
+
+The following modifications avoid this hurdle to migration.
+
+ 1. An implied import also brings old style implicits into scope. So, in Scala 3.0
+    an old-style implicit definition can be brought into scope either by a normal or
+    by an implied import.
+
+ 2. In Scala 3.1, an old-style implicits accessed implicitly through a normal import
+    will give a deprecation warning.
+
+ 3. In some version after 3.1, an old-style implicits accessed implicitly through a normal import
+    will give a compiler error.
+
+Thr rules mean that library users can use `import implied` to access old-style implicits in Scala 3.0,
+and will be gently nudged and then forced to do so in later versions. Libraries can then switch to
+implied instances once their user base has migrated.