Add a comment for translators to 'dpkg-query -l' header string

Reported-by: Ask Hjorth Larsen <asklarsen@gmail.com>

src/querycmd.c

@@ -109,6 +109,12 @@ list1package(struct pkginfo *pkg, bool *head, struct pkg_array *array)
   }
 
   if (!*head) {
+    /* TRANSLATORS: This is the header that appears on 'dpkg-query -l'. The
+     * string should remain under 80 characters. The uppercase letters in
+     * the state values denote the abbreviated letter that will appear on
+     * the first three columns, which should ideally match the English one
+     * (e.g. Remove → supRimeix), see dpkg-query(1) for further details. The
+     * translated message can use additional lines if needed. */
     fputs(_("\
 Desired=Unknown/Install/Remove/Purge/Hold\n\
 | Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend\n\