Improve handling of user settings in :TOhtml. Default to generating CSS.

(Benjamin Fritz)

runtime/doc/syntax.txt

@@ -423,10 +423,10 @@ Force to omit the line numbers by using a zero value: >
 Go back to the default to use 'number' by deleting the variable: >
    :unlet g:html_number_lines
 
-By default, HTML optimized for old browsers is generated.  If you prefer using
-cascading style sheets (CSS1) for the attributes (resulting in considerably
-shorter and valid HTML 4 file), use: >
-   :let g:html_use_css = 1
+By default, valid HTML 4.01 using cascading style sheets (CSS1) is generated.
+If you need to generate markup for really old browsers or some other user
+agent that lacks basic CSS support, use: >
+   :let g:html_use_css = 0
 
 Concealed text is removed from the HTML and replaced with the appropriate
 character from |:syn-cchar| or 'listchars' depending on the current value of
@@ -496,16 +496,18 @@ And to go back to displaying up to three lines again: >
 <
 					    *convert-to-XML* *convert-to-XHTML*
 An alternative is to have the script generate XHTML (XML compliant HTML).  To
-do this set the "use_xhtml" variable: >
-    :let use_xhtml = 1
-To disable it again delete the variable: >
-    :unlet use_xhtml
+do this set the "html_use_xhtml" variable: >
+    :let g:html_use_xhtml = 1
+
+Any of these options can be enabled or disabled by setting them explicitly to
+the desired value, or restored to their default by removing the variable using
+|:unlet|.
 
 Remarks:
 - This only works in a version with GUI support.  If the GUI is not actually
   running (possible for X11) it still works, but not very well (the colors
   may be wrong).
-- Older browsers will not show the background colors.
+- Some truly ancient browsers may not show the background colors.
 - From most browsers you can also print the file (in color)!
 
 Here is an example how to run the script over all .c and .h files from a