--allow-redefinition-new is no longer experimental (#21110)

@ilevkivskyi made a bunch of improvements to the feature.

Author: JukkaL

docs/source/command_line.rst

@@ -598,7 +598,7 @@ of the above sections.
 .. option:: --allow-redefinition-new
 
     By default, mypy won't allow a variable to be redefined with an
-    unrelated type. This flag enables the redefinition of unannotated
+    unrelated type. This flag enables the redefinition of *unannotated*
     variables with an arbitrary type. You will also need to enable
     :option:`--local-partial-types <mypy --local-partial-types>`.
     Example:
@@ -645,7 +645,6 @@ of the above sections.
 
     Note: We are planning to turn this flag on by default in a future mypy
     release, along with :option:`--local-partial-types <mypy --local-partial-types>`.
-    The feature is still experimental, and the semantics may still change.
 
 .. option:: --allow-redefinition