chore(docs): more info for the changelog

Signed-off-by: Henri Menke <henri@henrimenke.de>

doc/generic/pgf/CHANGELOG.md

@@ -16,6 +16,18 @@ lot of contributed changes. Thanks to everyone who volunteered their time!
   `\pgfrevision{,date}` and `\pgfversion{,date}` are synonyms for now, but the
   revision should eventually gain back its original meaning.  However, as of now
   this is not supported by l3build.
+- Many operations in `pgfkeys` used to use `\csname` directly which lets the
+  given csname become `\relax` in case it wasn't defined. This resulted in some
+  leakage of accidentally `\relax`ed keys into the global scope. This has been
+  cleaned up a little. To preserve compatibility macros that used to expand to a
+  `\relax`ed csname now expand to a primitive `\relax`. This affects the
+  user-level commands `\pgfkeysgetvalue` and `\pgfkeysgetvalueof`. For the
+  former the change should not be visible except for the number of expansions
+  required. For `\pgfkeysgetvalueof`, however, the behavior is manifestly
+  different in that it will now expand to a primitive `\relax` in case the value
+  is undefined instead of a `\relax`ed csname. It has always been semantically
+  wrong to assign to the result of `\pgfkeysgetvalueof`, but now it will have
+  undesired side-effects. Therefore this change is breaking.
 
 ### Added