attributes: document default level for err (#2433)

## Motivation

The current description for the default level of the `err` return value
event _strongly implies_ it's the same as the span. However, the
implementation actually defaults to `ERROR`.

## Solution

This PR documents that, so future generations don't have to chase down
the truth, like I did. 😉

tracing-attributes/src/lib.rs

@@ -463,6 +463,8 @@ mod expand;
 /// }
 /// ```
 ///
+/// The level of the error value event defaults to `ERROR`.
+///
 /// Similarly, overriding the level of the `err` event :
 ///
 /// ```