feature #5672 Add constants to BC promise (WouterJ)

This PR was submitted for the 2.8 branch but it was merged into the 2.7 branch instead (closes #5672).

Discussion
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Add constants to BC promise

| Q | A
| --- | ---
| Doc fix? | no
| New docs? | yes
| Applies to | all
| Fixed tickets | #5224

This does things a little bit different than proposed in #5224, based on [this comment by @fabpot](symfony/symfony#15680 (comment)) and the fact that one uses constants, to allow to change the value without breaking the code.

/cc @webmozart @stof @weaverryan @xabbuh please review this & give your opinion on this.

Commits
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d25c28d Add constants to BC promise

Author: weaverryan

contributing/code/bc.rst

@@ -165,6 +165,10 @@ Add type hint to an argument                    No
 Remove type hint of an argument                 No
 Change argument type                            No
 Change return type                              No
+**Constants**
+Add constant                                    Yes
+Remove constant                                 No
+Change value of a constant                      Yes [1]_ [5]_
 ==============================================  ==============
 
 Changing Classes
@@ -248,6 +252,10 @@ Change return type                                  Yes
 **Static Methods**
 Turn non static into static                         No
 Turn static into non static                         No
+**Constants**
+Add constant                                        Yes
+Remove constant                                     No
+Change value of a constant                          Yes [1]_ [5]_
 ==================================================  ==============
 
 .. [1] Should be avoided. When done, this change must be documented in the
@@ -262,6 +270,13 @@ Turn static into non static                         No
 .. [4] When changing the parent class, the original parent class must remain an
        ancestor of the class.
 
+.. [5] The value of a constant may only be changed when the constants aren't
+       used in configuration (e.g. Yaml and XML files), as these do not support
+       constants and have to hardcode the value. For instance, event name
+       constants can't change the value without introducing a BC break.
+       Additionally, if a constant will likely be used in objects that are
+       serialized, the value of a constant should not be changed.
+
 .. _Semantic Versioning: http://semver.org/
 .. _scalar type: http://php.net/manual/en/function.is-scalar.php
 .. _boolean values: http://php.net/manual/en/function.boolval.php