Improve documentation for html under Formats

[geraldfullam]

I discovered through trial that when using theo to build html that some categories of tokens were not rendered to the page. Upon examining the source code, I learned that these categories are conditionally rendered and that each is mapped to a human-friendly display name.

I propose mentioning this conditional rendering and explicitly listing the supported categories and their mapped display names (and perhaps in the future making this configurable via options).

Also, the comment in the example for passing additional options for the html format implies that there are more than just the one transformPropName shown, but upon examining the source code it appears that this is the only additional configurable option at this time.

I propose explicitly listing the configurable option(s) in the documentation.

[aputinski]

I totally agree. Would you mind submitting a PR?