libstdc++ pass 1: rephrase the explanation for --with-gxx-include-dir

I just received a private mail from some guy obviously puzzled by this
switch.  Expand the explanation a little to make it more clear.

1 files changed, 13 insertions, 3 deletions

diff --git a/chapter05/libstdc++.xml b/chapter05/libstdc++.xml
index 750868591..91f2b6b19 100644
--- a/chapter05/libstdc++.xml
+++ b/chapter05/libstdc++.xml
@@ -92,11 +92,21 @@ cd       build</userinput></screen>
       <varlistentry>
         <term><parameter>--with-gxx-include-dir=/tools/$LFS_TGT/include/c++/&gcc-version;</parameter></term>
         <listitem>
-          <para>This is the location where the C++ compiler should search for the
-          standard include files. In a normal build, this information
+          <para>This specifies the installation directory for include files.
+          Because libstdc++ is the standard C++ library for LFS, this
+          directory should match the location where the C++ compiler
+          (<command>$LFS_TGT-g++</command>) would search for the
+          standard C++ include files. In a normal build, this information
           is automatically passed to the libstdc++ <command>configure</command>
           options from the top level directory. In our case, this information
-          must be explicitly given.</para>
+          must be explicitly given.
+          The C++ compiler will prepend the sysroot path
+          <filename class="directory">$LFS</filename> (specified building
+          GCC pass 1) to the include file search path, so it will actually
+          search in
+          <filename class="directory">$LFS/tools/$LFS_TGT/include/c++/&gcc-version;</filename>.  The combination of the <parameter>DESTDIR</parameter>
+          variable (in the <command>make install</command> command below)
+          and this switch ensures to install the headers there.</para>
         </listitem>
       </varlistentry>