nixpkgs manual: unfree packages are untested

Resolves confusion mentioned in #31045.

Changed files
+4
doc
+4
doc/configuration.xml
···
</programlisting>
</para>
+
<para>Note that we are not able to test or build unfree software on Hydra
+
due to policy. Most unfree licenses prohibit us from either executing or
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distributing the software.</para>
+
<section xml:id="sec-allow-broken">
<title>Installing broken packages</title>