Removed buildout.dumppickedversions from the warning.

I *have* retained buildout.dumppickedversions in the doctest with
the qualification 'only part of it'.

Removing the warning *does* make it less obvious for the people
that use buildout.dumppickedversions in the way I thought it
was used most, but apparently the default behaviour of dumppickedversions
was otherwise.
Re-instated buildout.dumppickedversions into the warning.
Reworded the warning.
Merge pull request #83 from reinout/remove-dumppickedversions-note

Removed buildout.dumppickedversions from the warning. Partial fix for #70