Document pretty parameter for mon option

Documentation: document pretty parameter for mon option
that turns on JSON pretty printing
Signed-off-by: NVicente Jimenez Aguilar <googuy@gmail.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

qemu-options.hx

@@ -3487,11 +3487,12 @@ Like -qmp but uses pretty JSON formatting.
 ETEXI
 
 DEF("mon", HAS_ARG, QEMU_OPTION_mon, \
-    "-mon [chardev=]name[,mode=readline|control]\n", QEMU_ARCH_ALL)
+    "-mon [chardev=]name[,mode=readline|control][,pretty[=on|off]]\n", QEMU_ARCH_ALL)
 STEXI
-@item -mon [chardev=]name[,mode=readline|control]
+@item -mon [chardev=]name[,mode=readline|control][,pretty[=on|off]]
 @findex -mon
-Setup monitor on chardev @var{name}.
+Setup monitor on chardev @var{name}. @code{pretty} turns on JSON pretty printing
+easing human reading and debugging.
 ETEXI
 
 DEF("debugcon", HAS_ARG, QEMU_OPTION_debugcon, \