Make format args explicit in DocString.

Previously it was not clear that passing a single argument to
DocString.text, DocString.append, or DocString.uri would be treated as
a format string. With this change, the 'text', 'append', and 'uri'
methods take literal strings, and the new 'format', 'appendFormat',
and 'formattedUri' methods take format strings.

Bug: 23782192
Change-Id: I9a094575f0831de6659033052305f918c71ac8b7