qapi: better docs for calc-dirty-rate and friends

Rewrote calc-dirty-rate documentation. Briefly described
different modes of dirty page rate measurement. Added some
examples. Fixed obvious grammar errors.

Signed-off-by: Andrei Gudkov <gudkov.andrei@huawei.com>
Message-Id: <fe7d32a621ebd69ef6974beb2499c0b5dccb9e19.1684854849.git.gudkov.andrei@huawei.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
[Prose tweaked and spacing corrected, as per review]
Signed-off-by: Markus Armbruster <armbru@redhat.com>