docs: advice to resolve discussion on gitlab MR doc

docs: advice to resolve discussion on gitlab MR doc

For newcomers to gitlab, it is not evident that it is better to press
the "Resolve Discussion" button when you update your branch handling
feedback.

v2:
   * Fix several grammar nits, reorder, use new corrected text (Connor
     Abbot)
   * Use "reviewers", instead of reviewer (Eric Engestrom)

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>