doc/changes.src: document PTR warning

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
diff --git a/doc/changes.src b/doc/changes.src
index 1010409..ab43d60 100644
--- a/doc/changes.src
+++ b/doc/changes.src
@@ -17,6 +17,10 @@
 
 \b More Codeview debug format fixes.
 
+\b If the MASM PTR keyword is encountered, issue a warning.  This is
+   much more likely to indicate a MASM-ism encountered in NASM than it
+   is a valid label.  This warning can be suppressed with -w-ptr.
+
 
 \S{cl-2.12.01} Version 2.12.01