[TASK] Fixed wrong docblock.

[skip ci]
Grammar police
Move the /tmp/magento/var warning to stderr

This warning, while useful in interactive usage, complicates things in
script. Errors/warnings belong in stderr, especially if a command can
finish it's task succefully.

This fix was prompted by using n98 in a cron script, like so:
    #!/bin/sh
    # ...
    prefix=`n98-magerun.phar db:info prefix`
    n98-magerun.phar db:query "TRUNCATE ${prefix}core_url_rewrites;"
    # ...

This generated an SQL syntax error.

TODO:
- There may be more cases where warnings should be moved to stderr.
- Investigate if the formatter can be extended to automatically route
  <error> tag to OutputInterface::getErrorOutput() for a better fix.