dpkg-maintscript-helper: clarify meaning of lastversion for rm_conffile

Clarify that dpkg-maintscript-helper rm_conffile needs the last version of
the package that did not remove the obsolete conffile if this was not
implemented at the time the file became obsolete.

debian/changelog

@@ -21,6 +21,9 @@ dpkg (1.15.8) UNRELEASED; urgency=low
     compiling. Thanks to Loïc Minier for the initial patch. Closes: #578365
   * Make dpkg-maintscript-helper more robust when required parameters are
     missing. Closes: #582814
+  * Clarify that dpkg-maintscript-helper rm_conffile needs the last version of
+    the package that did not remove the obsolete conffile if this was not
+    implemented at the time the file became obsolete. Closes: #582893
 
   [ Guillem Jover ]
   * Require gettext 0.18:

man/dpkg-maintscript-helper.1

@@ -56,7 +56,9 @@ All of this is implemented by putting the following shell snippet in the
 .P
 \fIconffile\fP is the filename of the conffile to remove.
 \fIlastversion\fP is the last version of the package that contained the
-conffile. \fIpackage\fP is the package name, it's optional as it will
+conffile (or the last version of the package that did not take care to
+remove the obsolete conffile if this was not immediately implemented).
+\fIpackage\fP is the package name, it's optional as it will
 default to $DPKG_MAINTSCRIPT_PACKAGE (this variable is set by dpkg to the
 name of the package acted upon). All the parameters of the maintainer
 scripts have to be forwarded to the program after "\-\-".