Recent commit 3751d7c4 "vl: allow full-blown QemuOpts syntax for
-global" overloaded its existing argument syntax DRIVER.PROP=VALUE
with QemuOpts syntax.  Unambigious as long as no DRIVER contains '='.

Its documentation claims that "the two syntaxes are equivalent."
Improve it to spell out how exactly the old syntax gets desugared into
the new one.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>