Fix missing favicon.ico warning (Sphinx 1.2.2). The comments next to
``html_favicon`` seem to conflict with [what it in the sphinx
docs](http://sphinx-doc.org/config.html#confval-html_favicon).

So I've copied the comments from there to conf.py and changed its
value appropriately to remove the missing favicon.ico warning.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@214971 91177308-0d34-0410-b5e6-96231b3b80d8