add comment about interpolation

config/config.go

@@ -988,6 +988,7 @@ type RelabelConfig struct {
 	// Modulus to take of the hash of concatenated values from the source labels.
 	Modulus uint64 `yaml:"modulus,omitempty"`
 	// The label to which the resulting string is written in a replacement.
+	// regex interpolation is allowed for the replace action.
 	TargetLabel string `yaml:"target_label,omitempty"`
 	// Replacement is the regex replacement pattern to be used.
 	Replacement string `yaml:"replacement,omitempty"`