gcc-pass1: Explain -i.orig

A frequently asked question is "why not using -i.orig instead of -i for
${some_random_sed_command_in_book}" but I really expect people to learn
from this example and do it on their own if they really need...

chapter05/gcc-pass1.xml

@@ -72,6 +72,23 @@ mv -v mpc-&mpc-version; mpc</userinput></screen>
  ;;
 esac</userinput></screen>
 
+    <note>
+      <para>
+        This example demonstrates the use of the
+        <parameter>-i.orig</parameter> switch.  It makes the
+        <command>sed</command> copy the <filename>t-linux64</filename>
+        to <filename>t-linux64.orig</filename>, and then edit the
+        <filename>t-linux64</filename> inplace.  So you may run
+        <command>diff -u gcc/config/i386/t-linux64{.orig,}</command>
+        to visualize the change done by the <command>sed</command> command
+        afterwards.  We'll simply use <parameter>-i</parameter> (which just
+        edits the original file inplace without copying it) for all other
+        packages in the book, but you can change it to
+        <parameter>-i.orig</parameter> in any case you want to keep a copy
+        of the original file.
+      </para>
+    </note>
+
     <para>The GCC documentation recommends building GCC
     in a dedicated build directory:</para>