docs: Update description of 'user=username' for '-run-with'

docs: Update description of 'user=username' for '-run-with'

The description of '-runas' and '-run-with' didn't explain that QEMU
will use setuid/setgid to implement the option, so the user might get
confused if using 'elevateprivileges=deny' as well.

Since '-runas' is going to be deprecated and replaced by '-run-with'
in the coming qemu9.1, add the message there.

Signed-off-by: Boqiao Fu <bfu@redhat.com>
Link: https://lore.kernel.org/r/CAFRHJ6J9uMk+HMZL+W+KE1yoRCOLPgbPUVVDku55sdXYiGXXHg@mail.gmail.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>