go/analysis/passes/composite: add an example to the doc

Change-Id: I929a5390aa5f4a4484e57ffa1eaa66f3238da1c1
Reviewed-on: https://go-review.googlesource.com/c/159338
Reviewed-by: Alan Donovan <adonovan@google.com>

go/analysis/passes/composite/composite.go

@@ -21,7 +21,16 @@ const Doc = `check for unkeyed composite literals
 This analyzer reports a diagnostic for composite literals of struct
 types imported from another package that do not use the field-keyed
 syntax. Such literals are fragile because the addition of a new field
-(even if unexported) to the struct will cause compilation to fail.`
+(even if unexported) to the struct will cause compilation to fail.
+
+As an example,
+
+	err = &net.DNSConfigError{err}
+
+should be replaced by:
+
+	err = &net.DNSConfigError{Err: err}
+`
 
 var Analyzer = &analysis.Analyzer{
 	Name:             "composites",