fix: correct README examples and add missing godoc on WithUnimplemented

Two README examples used 'return nil, fault' with a single-error return
type. Add missing doc comment on WithUnimplemented to match all other
With* functions.

Author: basgys

README.md

@@ -57,7 +57,7 @@ This error indicates that the request does not have valid authentication credent
 func SensitiveOperation(ctx context.Context) error {
   user, ok := user.FromContext(ctx)
   if !ok {
-    return nil, faults.Unauthenticated
+    return faults.Unauthenticated
   }
 
   // Perform operation
@@ -147,10 +147,10 @@ It must not be used for rejections caused by exhausting some resource. It must a
 func SensitiveResource(ctx context.Context) error {
   user, ok := user.FromContext(ctx)
   if !ok {
-    return nil, faults.Unauthenticated
+    return faults.Unauthenticated
   }
   if !user.IsAdmin() {
-    return nil, faults.PermissionDenied
+    return faults.PermissionDenied
   }
 
   // Perform operation

faults.go

@@ -89,6 +89,7 @@ func WithResourceExhausted(parent error, violations ...*QuotaViolation) error {
 	return &QuotaFailure{parent, violations}
 }
 
+// WithUnimplemented wraps `parent` with an `UnimplementedFailure`
 func WithUnimplemented(parent error) error {
 	return &UnimplementedFailure{parent}
 }