doc: Document forthcomming NIX_DEBUG degredations

doc/stdenv.xml

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-
 <chapter xmlns="http://docbook.org/ns/docbook"
          xmlns:xlink="http://www.w3.org/1999/xlink"
          xml:id="chap-stdenv">
···
 
   <varlistentry>
     <term><varname>NIX_DEBUG</varname></term>
-    <listitem><para>If set, <literal>stdenv</literal> will print some
-    debug information during the build.  In particular, the
-    <command>gcc</command> and <command>ld</command> wrapper scripts
-    will print out the complete command line passed to the wrapped
-    tools.</para></listitem>
+    <listitem><para>
+      A natural number indicating how much information to log.
+      If set to 1 or higher, <literal>stdenv</literal> will print moderate debug information during the build.
+      In particular, the <command>gcc</command> and <command>ld</command> wrapper scripts will print out the complete command line passed to the wrapped tools.
+      If set to 6 or higher, the <literal>stdenv</literal> setup script will be run with <literal>set -x</literal> tracing.
+      If set to 7 or higher, the <command>gcc</command> and <command>ld</command> wrapper scripts will also be run with <literal>set -x</literal> tracing.
+    </para></listitem>
   </varlistentry>
 
 </variablelist>