Update compress.xml

added remark treshold

opcodes/compress.xml

@@ -63,7 +63,7 @@ amplitude of 0dbfs.
   <refsect1>
     <title>Performance</title>
     <para>
-      <emphasis>kthresh</emphasis> -- sets the lowest decibel level that will be allowed through.  Normally 0 or less, but if higher the threshold will begin removing low-level signal energy such as background noise.
+      <emphasis>kthresh</emphasis> -- sets the lowest decibel level that will be allowed through. This is a threshold of a separate noise gate, normally 0 or less, but if higher the threshold will begin removing low-level signal energy such as background noise.
     </para>
     <para>
       <emphasis>kloknee</emphasis>, <emphasis>khiknee</emphasis> -- decibel break-points denoting where compression or expansion will begin. These set the boundaries of a soft-knee curve joining the low-amplitude 1:1 line and the higher-amplitude compression ratio line.  Typical values are 48 and 60 db.   If the two breakpoints are equal, a hard-knee (angled) map will result.