document that `.delete` does work on `#readonly?` records. Closes #11860

[ci skip]

This is due to the fact that `.delete` is directly translated to SQL.
It tries to follow the same rules as `.delete_all` which is not able
to verify that records are `#readonly?`.

activerecord/lib/active_record/persistence.rb

@@ -149,6 +149,8 @@ def save!(*)
     # The row is simply removed with an SQL +DELETE+ statement on the
     # record's primary key, and no callbacks are executed.
     #
+    # Note that this will also delete records marked as <tt>readonly?</tt>.
+    #
     # To enforce the object's +before_destroy+ and +after_destroy+
     # callbacks or any <tt>:dependent</tt> association
     # options, use <tt>#destroy</tt>.