man page is missing suboptions for "-display"

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1620660

Some of the display options have suboptions, for example:

> -display gtk[,grab_on_hover=on|off][,gl=on|off]

None of these suboptions are currently documented in qemu-options.hx
(checked git@f04ec5a)

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