Added more explanations about the differences between HTML and XML parsing modes.

lib/python/Products/PageTemplates/help/PageTemplate_Edit.stx

@@ -21,7 +21,11 @@ Edit View: Edit A Page Template
         content type is 'text/html' (the default) then the template is
         assumed to contain HTML, not XML.  This affects both parsing
         and rendering of the template, and can be overridden by giving
-        the template an 'is_html' property.
+        the template an 'is_html' property. In HTML mode you don't
+        need to explicitly declare 'tal' and 'metal' XML namespaces. In
+        XML mode you must explicitly declare 'tal' and 'metal' XML
+        namespaces, and Zope assumes that your template contains
+        well-formed XML.
 
         'Expand macros when editing' -- Allows you to turn the expansion
         of METAL macros on or off.  This only affects viewing of the

lib/python/Products/PageTemplates/help/metal.stx

@@ -22,6 +22,10 @@ METAL Overview
     Just like the TAL namespace URI, this URI is not attached to a web
     page; it's just a unique identifier.
 
+    Zope does not require an XML namespace declaration when creating
+    templates with a content-type of 'text/html'. However, it does
+    require an XML namespace declaration for all other content-types.
+
   METAL Statements 
 
     METAL defines a number of statements:

lib/python/Products/PageTemplates/help/tal.stx

@@ -26,6 +26,10 @@ TAL Overview
     This is not a URL, but merely a unique identifier.  Do not expect
     a browser to resolve it successfully.
 
+    Zope does not require an XML namespace declaration when creating
+    templates with a content-type of 'text/html'. However, it does
+    require an XML namespace declaration for all other content-types.
+
   TAL Statements   
 
     These are the tal statements: