CIRC_BW fields vague

Hi lovely network team folks. Yesterday Mike documented CIRC_BW's new fields...

https://gitweb.torproject.org/torspec.git/commit/?id=fbb38ec

This is great, but this isn't quite enough for me to grok what they are. "Bytes read and written on a circuit" is simple and clear, but "the total length of the unused overhead portion of valid relay cells" doesn't make sense to me.

I'll submit a patch in a minute I find clearer giving a rough guess at what I think these are, but my description will no doubt be inaccurate. Hopefully together we'll be able to come up with something both clear and correct. :)