dpkg(1): Document available database update procedure to set selections

Mention that the available database needs to be updated, refer to the
dpkg FAQ for more details, and add an example when using apt.

Suggested-by: Klaus Ita <koki.eml@gmail.com>

debian/changelog

@@ -2,6 +2,8 @@ dpkg (1.17.1) UNRELEASED; urgency=low
 
   * Man pages cleanup:
     - Make explicit that dpkg-shlibdeps -e can be used multiple times.
+    - Improve documentation on how to update the available database before
+      setting package selections. Suggested by Klaus Ita <koki.eml@gmail.com>.
   * Add manpages-it Replaces to dpkg due to new translated man pages.
     Reported by Alessio Gaeta <alga777@gmail.com>. Closes: #717983
   * Reword libdpkg-perl description and only list public modules.

man/dpkg.1

@@ -23,7 +23,7 @@
 .\" You should have received a copy of the GNU General Public License
 .\" along with this program.  If not, see <http://www.gnu.org/licenses/>.
 .
-.TH dpkg 1 "2013-04-05" "Debian Project" "dpkg suite"
+.TH dpkg 1 "2013-07-28" "Debian Project" "dpkg suite"
 .SH NAME
 dpkg \- package manager for Debian
 .
@@ -800,8 +800,16 @@ To make a local copy of the package selection states:
 \fB     dpkg \-\-get\-selections >myselections\fP
 .br
 
-You might transfer this file to another computer, and install it there
-with:
+You might transfer this file to another computer, and after having updated
+the available database there with your package manager frontend of choice
+(see https://wiki.debian.org/Teams/Dpkg/FAQ for more details), for example:
+.br
+\fB     avail=`mktemp`\fP
+\fB     apt\-cache dumpavail >"$avail"\fP
+\fB     dpkg \-\-merge\-avail "$avail"\fP
+\fB     rm "$avail"\fP
+.br
+you can install it with:
 .br
 \fB     dpkg \-\-clear\-selections\fP
 \fB     dpkg \-\-set\-selections <myselections\fP