docs: Update docs for arbitrary port names

- Update sample application section to take into account changes made in
  arbitrary port names.
- Small formatting and grammatical corrections

Change-Id: I7056bd073711053a490501e7e9e157da300a2f49
Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-by: Eimear Morrissey <eimear.morrissey@intel.com>