Note that Repository#Flush ignores cache prefix

As discussed in https://github.com/laravel/framework/issues/5034

This patch adds a warning in the docstring for the flush method and also in the help text of the `artisan cache:flush` command.

If you would prefer this to be against a different branch I can rebase.

Sister pull request to https://github.com/laravel/laravel/pull/3199
Please use single quotes.