Add a @fixme comment on FormatJson::encode(); the '$isHtml' parameter's purpose and...

Add a @fixme comment on FormatJson::encode(); the '$isHtml' parameter's purpose and function is not documented, but it seems to map to a pretty-printing parameter on Services_JSON(). If that's its purpose, it should be renamed and documented. Since it's a bare boolean parameter with no clear purpose in calls when "true" or "false" is passed bare, it probably also wants a nice legible constant value or two.