Neither Drmanu's nor Gilla's answer is quite right. This species has two names, so this is its correct botanical designation. syn. is short for synonym, and it's tingitanus (= of Tangiers), not tangitanus. The R. is to avoid repeating Rumex, and Tangier sorrel is the common name.

I think that in the asker's context, which sounds like it's meant for the general reader and is not heavily botanical, I'd just use one of the two Latin names with the common name in brackets.