Template:Fix-span/doc

This meta-template is just like fix with three exceptions:
 * It takes a content parameter containing wikitext to be placed within a  element before the superscripted notice.
 * It takes a span-class parameter specifying the HTML class name for the  element.
 * It gives special priority over text rather than the other way around.

The spanned content is displayed like this.

This is a separate template for now because this is an idea imported from the Italian CorporisPublica (q.v.). Eventually, if this idea takes off, this mechanism could be folded into fix.