Change the font of the help, just size can be modified.

This is true for html files that are styled thru style sheets.
We could implement a workaround using jQuery and manipulate
the html/ css, but this is so slow that I won't do that.

Instead add a note and tell the user about the underlying problem.

Task-number: QTCREATORBUG-4871
Change-Id: Id3d0a490bb160825ec643f8c84871aad6cf1b796
Reviewed-by: Niels Weber <niels.2.weber@nokia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@nokia.com>