Docs/bpf: Improve bpf_design_QA.rst

- Rephrase wording such that the file flows better
- Improve consistency with respect to word use, grammar and style
- Remove point on loops not being supported as BPF now has support for
  bounded loops

Signed-off-by: Rhys Rustad-Elliott <me@rhysre.net>

Author: GunshipPenguin