[RN][NFC] Document behaviour change of dictionary generator

in presence of selected template instances with template arguments
that are represented in the AST as invalid.

Author: dpiparo

README/ReleaseNotes/v638/index.md

@@ -4,6 +4,9 @@
 * The `RooStats::HLFactory` class that was deprecated in ROOT 6.36 is now removed. It provided little advantage over using the RooWorkspace directly or any of the other higher-level frameworks that exist in the RooFit ecosystem.
 * The build options `mysql`, `odbc` and `pgsql`, that were deprecated in ROOT 6.36, are now removed.
 
+## Core Libraries
+* Behavior change: when selecting a template instantiation for a dictionary, all the template arguments have to be fully defined - the forward declarations are not enough any more. The error prompted by the dictionary generator will be `Warning: Unused class rule: MyTemplate<MyFwdDeclaredClass>`.
+
 ## Math
 
 ### Minuit2