Add a warning about mode being octal

lib/ansible/utils/module_docs_fragments/files.py

@@ -25,7 +25,7 @@ options:
     required: false
     default: null
     description:
-      - mode the file or directory should be, such as 0644 as would be fed to I(chmod). As of version 1.8, the mode may be specified as a symbolic mode (for example, C(u+rwx) or C(u=rw,g=r,o=r)).
+      - mode the file or directory should be. For those used to I(/usr/bin/chmod) remember that modes are actually octal numbers (like 0644). Leaving off the leading zero will likely have unexpected results. As of version 1.8, the mode may be specified as a symbolic mode (for example, C(u+rwx) or C(u=rw,g=r,o=r)).
   owner:
     required: false
     default: null