.doc files are microsoft specific files. That is, they were made by MS as a format for their Word (proccessor) program. They way they represent their documents is quite complex, and there is no easy way to automate the proccess of converting from HTML to doc format.
You could, however, consider converting to a more broadly used format (eg RTF or rich text format). While some of the formatting you would find in word may not be available in RTF, as far as I know a standard (and therefore a clear definition) for it exists <pls correct me if I'm wrong, those who know better>. The other option, might be to consider using xml, which (while I have limited exposure to) is a standard which is hoped (some day) to standardise documents (and specifically their encoding).
SHORT ANSWER: I doubt MS are too keen to explain exactly how they code doc's. Other Documents can be opened using their programs, however, and would certainly be simpler and probably more portable than the doc format.
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