document --no- as --show-upgraded is the default

--show-upgraded is the default since
906fbf8886926eeb302332d997c9bd861291e155 so documenting it as if it
would be an option having an effect as is feels wrong and we do the same
for other options like install-recomends, download, …, too.

This commit also removes -u from the documentation, but still supports
it in the commandline parsing. Eventually we should deprecate the short
option, but for now lets just stop documenting it.

Closes: 824456

doc/apt-get.8.xml

@@ -357,9 +357,8 @@
      Configuration Item: <literal>APT::Get::Assume-No</literal>.</para></listitem>
      </varlistentry>
 
-     <varlistentry><term><option>-u</option></term><term><option>--show-upgraded</option></term>
-     <listitem><para>Show upgraded packages; print out a list of all packages that are to be
-     upgraded. 
+     <varlistentry><term><option>--no-show-upgraded</option></term>
+     <listitem><para>Do not show a list of all packages that are to be upgraded.
      Configuration Item: <literal>APT::Get::Show-Upgraded</literal>.</para></listitem>
      </varlistentry>