It is available now on http://dehs.alioth.debian.org the html version of - man uscan - that describes the Watch file format.
Dehs, for upstream version, uses an hacked version (but 100% error message and watch format compatible) of uscan (you can find this tool in devscripts debian package). This because we have not a policy entry about Watch file format so uscan is the de-facto standard for Watch file format. The Watch file concept was introduced by some tools in devscripts. Actually the limit is that "man uscan" is the only standard about Watch file. Did we (Debian Community) want to create/find a standard for an official Watch file format for debian packages? I will adapt dehs to following this official format if we find one. P.s. you can alway start from the wizard generated watch file to create you own. Cheers, Stefano