Accurately reflect MSC-only for stage 1 metadex

Craig asked me to create this pull request to more accurately reflect what we are doing in the first iteration.
> Although the formatting of this message technically allows trading between any two currencies/properties, the parsing engines must require that either the currency id for sale or the currency id desired be Mastercoins ...

This is sort of like saying "even if the currency identifier field technically allows to be filled with negative numbers of never ever existing properties, the parsing engines must.." - might as well be more explicit and say what this actually means, e.g. "one of the traded currency pairs must be Mastercoin", and I'd leave out implementation hints such the mention of a parsing engine.