Typos/Atom-based Double-bond rewording

Hi Tim,

Fixed two typos in the specification. I've also significantly re-worded the atom-based double bond configuration. There was a number of inaccuracies and the theoretical illegal example wasn't actually illegal (Noel confirmed this on the mailing list). There is a theoretical problem though which I've expanded upon - wasn't sure what you 
used to generate the depictions and have left that out.

Thanks,
J