docs: Update for Xen boot parameter changes.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

docs/src/user.tex

@@ -4088,6 +4088,8 @@ editing \path{grub.conf}.
   a list of pages not to be allocated for use because they contain bad
   bytes. For example, if your memory tester says that byte 0x12345678
   is bad, you would place `badpage=0x12345' on Xen's command line.
+\item [ serial\_tx\_buffer=$<$size$>$ ] Size of serial transmit
+  buffers. Default is 16kB.
 \item [ com1=$<$baud$>$,DPS,$<$io\_base$>$,$<$irq$>$
   com2=$<$baud$>$,DPS,$<$io\_base$>$,$<$irq$>$ ] \mbox{}\\
   Xen supports up to two 16550-compatible serial ports.  For example: