Added a para about replaceable text.

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@3959 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

prologue/typography.xml

@@ -46,4 +46,9 @@ the file <filename>$LFS/etc/group</filename> from whatever is typed on the follo
 the sequence EOF is encountered. Therefore, this whole section is generally 
 typed as seen.</para></blockquote>
 
+<para><replaceable>[REPLACED TEXT]</replaceable></para>
+
+<blockquote><para>This form of text is used to encapsulate text that is
+not to be typed as seen or copy and pasted.</para></blockquote>
+
 </sect1>