[#84] Clarify FilePath attribute format

Signed-off-by: Stanislav Bogatyrev <s.bogatyrev@yadro.com>

object/types.proto

@@ -144,8 +144,8 @@ message Header {
   //   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
+  //   Full path to be associated with the object on saving. Should use '/' as a
-  //   '/' and use '/' as a delimiting symbol. Trailing '/' should be
+  //   delimiting symbol. Should not have leading `/`. Trailing '/' should be
   //   interpreted as a virtual directory marker. If an object has conflicting
   //   FilePath and FileName, FilePath should have higher priority, because it
   //   is used to construct the directory tree. FilePath with trailing '/' and