Add extbrowser4, extbrowser5, ..., and extbrowser9

doc/MANUAL.html

@@ -204,9 +204,9 @@ bindings, see <a href="#Key:lynx">the Lynx-like key binding</a>.
 <TR><TD>C-h<TD>View history of URL
 <TR><TD>F<TD>Render frame
 <TR><TD>M<TD>Browse current document using external browser
-(use 2M and 3M to invoke second and third browser)
+(use 2M, 3M, ..., and 9M to invoke second, third, ..., and ninth browser)
 <TR><TD>ESC M<TD>Browse link using external browser
-(use 2ESC M and 3ESC M to invoke second and third browser)
+(use 2ESC M, 3ESC M, ..., and 9ESC M to invoke second, third, ..., and ninth browser)
 </table>
 
 <H3>File/Stream operation</H3>