general-concepts/licenses: Licenses must be plain text

Historically we had some PDF files in the repository, but the last
of them was removed in 2011.

Signed-off-by: Ulrich Müller <ulm@gentoo.org>

general-concepts/licenses/text.xml

@@ -232,9 +232,9 @@ indicates <c>GPL-2+</c> license:
 <p>
 If your package's license is not already in the tree, you must add the
 license before committing the package. When adding the license, use a
-plain text file (UTF-8 encoded) if at all possible. Some licenses are
-PDF files rather than plain text <d/> this should only be done if we
-are legally required to do so.
+plain text file (UTF-8 encoded), because non-text files
+<uri link="::general-concepts/tree/#What Belongs in the Tree?">do not belong
+in the repository</uri>.
 Finally you need to check if your license should be added to a license group
 as listed in $PORTDIR/profiles/license_groups. For more information see the
 <uri link="https://www.gentoo.org/glep/glep-0023.html">GLEP 23</uri>.