revert accidental removal of documentation for trusted option in sources.list

In b0d408547734100bf86781615f546487ecf390d9 I accidently removed the
documentation for Trusted and replaced it with Signed-By instead of
adding it.

Git-Dch: Ignore

doc/sources.list.5.xml

@@ -265,6 +265,20 @@ deb-src [ option1=value1 option2=value2 ] uri suite [component1] [component2] [.
        anomalies.
 
        <itemizedlist>
+	  <listitem><para><option>Trusted</option> (<option>trusted</option>)
+		is a tri-state value which defaults to APT deciding if a source
+		is considered trusted or if warnings should be raised before e.g.
+		packages are installed from this source. This option can be used
+		to override this decision either with the value <literal>yes</literal>,
+		which lets APT consider this source always as a trusted source
+		even if it has no or fails authentication checks by disabling parts
+		of &apt-secure; and should therefore only be used in a local and trusted
+		context (if at all) as otherwise security is breached. The opposite
+		can be achieved with the value no, which causes the source to be handled
+		as untrusted even if the authentication checks passed successfully.
+		The default value can't be set explicitly.
+	  </para></listitem>
+
 	  <listitem><para><option>Signed-By</option> (<option>signed-by</option>)
 		is either an absolute path to a keyring file (has to be
 		accessible and readable for the <literal>_apt</literal> user,