clarified NEWS: document_files are officially required since Isabelle2014, but the absence was tolerated as legacy feature;

--- a/NEWS	Fri May 22 18:13:31 2015 +0200
+++ b/NEWS	Sat May 23 17:19:37 2015 +0200
@@ -133,6 +133,11 @@
 antiquotations need to observe the margin explicitly according to
 Thy_Output.string_of_margin. Minor INCOMPATIBILITY.
 
+* Specification of 'document_files' in the session ROOT file is
+mandatory for document preparation. The legacy mode with implicit
+copying of the document/ directory is no longer supported. Minor
+INCOMPATIBILITY.
+
 
 *** Pure ***