Merge branch 'jt/trace-error-on-warning'

Like die() and error(), a call to warning() will also trigger a
trace2 event.

* jt/trace-error-on-warning:
  usage: add trace2 entry upon warning()

Documentation/technical/api-trace2.txt

@@ -466,7 +466,7 @@ completed.)
 
 `"error"`::
 	This event is emitted when one of the `error()`, `die()`,
-	or `usage()` functions are called.
+	`warning()`, or `usage()` functions are called.
 +
 ------------
 {