dpkg(1): further improve explanation about conffiles

dpkg(1): further improve explanation about conffiles

* man/dpkg.1: Refer to configuration files in the explanation about
package's states because all configuration files (conffiles and
non-conffiles) should be properly handled by the packages. conffiles are
handled by dpkg itself, and non-conffile configuration files by the
configuration scripts. Explain this in the paragraph about removal
of packages.