[#77] object: Add a refinement to description on the FileName attribute

Signed-off-by: Roman Loginov <r.loginov@yadro.com>

object/types.proto

@@ -141,7 +141,8 @@ message Header {
   // * Name \
   //   Human-friendly name
   // * FileName \
-  //   File name to be associated with the object on saving
+  //   File name to be associated with the object on saving. FileName must not
+  //   contain the delimiting symbol '/'.
   // * FilePath \
   //   Full path to be associated with the object on saving. Should start with a
   //   '/' and use '/' as a delimiting symbol. Trailing '/' should be