Add guideline that changing default values is a BC break

Author: OskarStark

CLAUDE.md

@@ -0,0 +1 @@
+- Always remove trailing whitespace

contributing/code/bc.rst

@@ -264,6 +264,7 @@ Add constructor without mandatory arguments                               Yes
 Add argument with a default value                                         Yes             :ref:`[11] <note-11>`
 Remove argument                                                           No              :ref:`[3] <note-3>`
 Add default value to an argument                                          Yes
+Change default value of an argument                                       No              :ref:`[12] <note-12>`
 Remove default value of an argument                                       No
 Add type hint to an argument                                              No
 Remove type hint of an argument                                           Yes
@@ -490,6 +491,11 @@ code when upgrading to newer Symfony versions.
 
 **[11]** Only optional argument(s) of a constructor at last position may be added.
 
+.. _note-12:
+
+**[12]** Changing a default value is a BC break because it changes the behavior
+of existing code that relies on the current default value.
+
 Making Code Changes in a Backward Compatible Way
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~