Clarify when FromStrErr is used.

src/lib.rs

@@ -231,6 +231,9 @@ impl Display for InvalidInput {
 impl Error for InvalidInput {}
 
 /// An error when attempting to create a value from a string via the `FromStr` trait.
+///
+/// This error type is only used for simple enums with unit variants. Event deserialization through
+/// the `FromStr` trait returns an `InvalidEvent` on error.
 #[derive(Clone, Copy, Eq, Debug, Hash, PartialEq)]
 pub struct FromStrError;