dpkg(1): Clarify that the «dpkg -l» example only lists installed packages

Also add a reference to «dpkg-query --load-avail» as a way to list
available packages.

Closes: #673305

debian/changelog

@@ -39,6 +39,9 @@ dpkg (1.16.4) UNRELEASED; urgency=low
     Thanks to Roger Leigh <rleigh@debian.org>.
   * Add support for “none” as a valid dpkg-deb compression strategy value.
     Closes: #674711
+  * Clarify in dpkg(1) that the «dpkg -l» example only lists installed
+    packages, and that to list available packages «dpkg-query --load-avail»
+    has to be used instead. Closes: #673305
 
   [ Updated man page translations ]
   * German (Helge Kreutzmann).

man/dpkg.1

@@ -757,7 +757,10 @@ Defined by \fBdpkg\fP on the maintainer script environment to the
 name of the script running (preinst, postinst, prerm, postrm).
 .
 .SH EXAMPLES
-To list packages related to the editor \fBvi\fP(1):
+To list installed packages related to the editor \fBvi\fP(1) (note that
+\fBdpkg\-query\fP does not load the available file anymore by default, and
+the \fBdpkg\-query\fP \fB\-\-load\-avail\fP option should be used instead
+for that):
 .br
 \fB     dpkg \-l \(aq*vi*\(aq\fP
 .br