KVM: doc: Fix incorrect word ordering regarding supported use of APIs

Per Paolo[1], instantiating multiple VMs in a single process is legal;
but this conflicts with KVM's API documentation, which states:

  The only supported use is one virtual machine per process, and one
  vcpu per thread.

However, an earlier section in the documentation states:

   Only run VM ioctls from the same process (address space) that was used
   to create the VM.

and:

   Only run vcpu ioctls from the same thread that was used to create the
   vcpu.

This suggests that the conflicting documentation is simply an incorrect
ordering of of words, i.e. what's really meant is that a virtual machine
can't be shared across multiple processes and a vCPU can't be shared
across multiple threads.

Tweak the blurb on issuing ioctls to use a more assertive tone, and
rewrite the "supported use" sentence to reference said blurb instead of
poorly restating it in different terms.

Opportunistically add missing punctuation.

[1] https://lkml.kernel.org/r/f23265d4-528e-3bd4-011f-4d7b8f3281db@redhat.com

Fixes: 9c1b96e3

 ("KVM: Document basic API")
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
[Improve notes on asynchronous ioctl]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>