Eliminate the defunct :cat_old_format warning type.

The support for the deprecated old format was removed a while ago,
but the warning type was forgotten to be removed, so it hanged along
without having any actual effect.
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.
Add parameters to :tag warning type.

Allows one to supply a list of unknown tag names on which the
warning is not shown.

Fixes #500
Fix +/- signs in front of warning types in help.

Quite a hack... something better is surely needed.