Add note on vec_perm invalid usage. Remove app_a.xml reference.

Signed-off-by: Bill Schmidt  <wschmidt@linux.ibm.com>

Intrinsics_Reference/bk_main.xml

@@ -123,7 +123,7 @@
   <xi:include href="ch_vec_reference.xml"/>
   <!-- xi:include href="ch_scal_reference.xml"/ -->
   <xi:include href="ch_isa_intrin_xref.xml"/>
-  <xi:include href="app_a.xml"/>
+  <!-- xi:include href="app_a.xml"/ -->
 
   <!-- The app_foundation.xml appendix file is required by all documents. -->
   <xi:include href="../../Docs-Master/common/app_foundation.xml"/>

Intrinsics_Reference/ch_vec_reference.xml

@@ -24033,6 +24033,14 @@ xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="section_vec_intrinsics">
 	    role="bold">c</emphasis>.
 	  </para>
 	</listitem>
+	<listitem>
+	  <para>
+	    The <code>vec_perm</code> built-in should only use permutations
+	    that reorder vector elements of the specified type, not to reorder
+	    bytes within those elements.  The results are not guaranteed to be
+	    consistent across big- and little-endian if you violate this rule.
+	  </para>
+	</listitem>
 	<listitem>
 	  <para>
 	    No Power compilers yet support the vector _Float16 type, so that