Eliminate Logger#warn_nodoc method.

Change the #warn method so it can be used for also passing in
arguments, and then use it for reporting the :nodoc warning too.
For this to work:

- Logger#warn expects a `file` parameter instead of `filename` and
  `linenr`, which must always be a hash.
- The fourth parameter is then free to be used as optional `args`
  array for the warning types that need it.