Document shared debug input and reporting mode

command.go

@@ -95,7 +95,8 @@ type FlagMeta struct {
 	// Name is a nonempty long name without leading dashes. It consists of
 	// Unicode lowercase letters or digits in segments separated by single
 	// hyphens; digit-leading and numeric-only names are valid. The names help,
-	// quiet, verbose, debug, progress and format are reserved.
+	// quiet, verbose, debug, progress and format are reserved, except that a
+	// BoolFlag named debug attaches to the shared reporting flag.
 	Name string
 	// Short is an optional one-byte spelling other than '-' or the reserved 'h'.
 	Short string