In order not to compromise QMP adoption and make users' life easier,
this commit adds a simple text documentation which fully describes
all QMP supported commands.
This is not ideal for a number of reasons (harder to maintain,
text-only, etc) but does improve the current situation.
Even if it's temporary - maintaining it in a separate file looks rather
unhandy.
Can't we generate the per-command documentation snippets also from
qemu-monitor.hx and merge them with a header/footer into some text file?
That .hx file is the one anyone adding/converting commands has to touch
anyway.