qemu-options: Improve readability of SMP related Docs

qemu-options: Improve readability of SMP related Docs

We have a description in qemu-options.hx for each CPU topology
parameter to explain what it exactly means, and also an extra
declaration for the target-specific one, e.g. "for PC only"
when describing "dies", and "for PC, it's on one die" when
describing "cores".

Now we are going to introduce one more non-generic parameter
"clusters", it will make the Doc less readable and  if we still
continue to use the legacy way to describe it.

So let's at first make two tweaks of the Docs to improve the
readability and also scalability:
1) In the -help text: Delete the extra specific declaration and
   describe each topology parameter level by level. Then add a
   note to declare that different machines may support different
   subsets and the actual meaning of the supported parameters
   will vary accordingly.
2) In the rST text: List all the sub-hierarchies currently
   supported in QEMU, and correspondingly give an example of
   -smp configuration for each of them.

Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211228092221.21068-2-wangyanan55@huawei.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>