* Separate superscript footnotes with a comma, otherwise it's a 2-digit number

* Separate superscript footnotes with a comma, otherwise it's a 2-digit number
* Make a URL into an HTML link using anchor tag
* Put code in fixed-width font
* Since the MinGW footnote is significantly more than the rest of the footnotes
  combined, it needs to have a border and background color to separate it from
  the regular flow of text for readability purposes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@20093 91177308-0d34-0410-b5e6-96231b3b80d8