libaom: Clarify how to use NASM

And mark it as nodep so jhalfs won't install both yasm and nasm with no
reason.

multimedia/libdriv/libaom.xml

@@ -72,8 +72,10 @@
 
     <bridgehead renderas="sect4">Recommended</bridgehead>
     <para role="recommended">
-      <xref linkend="nasm"/> or
       <xref linkend="yasm"/>
+      (or <xref role='nodep' linkend="nasm"/>, pass the
+      <option>-DENABLE_NASM=yes</option> option to <command>cmake</command>
+      if you want to use it instead of yasm)
     </para>
 
     <bridgehead renderas="sect4">Optional</bridgehead>