Add doc string for format method

src/de/mod.rs

@@ -114,6 +114,11 @@ pub trait Deserializer {
         self.visit(visitor)
     }
 
+    /// Specify a format string for the deserializer.
+    ///
+    /// The deserializer format is used to determine which format
+    /// specific field attributes should be used with the
+    /// deserializer.
     fn format() -> &'static str {
         ""
     }