Document how users can customize HTML report from XSpec?

[galtm]

Although the environment variable and the Ant property are now tested, I'm keeping them undocumented for now because they have not been reviewed.

Originally posted by @AirQuick in #818 (comment)

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This capability looks useful. Do we want to document it and make report customizability an official feature? If so, we should also make sure the XML result document uses a format/schema that we're happy with and don't plan to overhaul in the near future; communicate about the stability level of whatever code a report-customizing user would be building on top of; and consider documenting the XML result document format and/or its recommended extension points.