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>

docs/submittingpatches.html

@@ -258,6 +258,9 @@ your email administrator for this.)
 </p>
 <ul>
   <li>Make changes and update your branch based on feedback
+  <li>After an update, for the feedback you handled, close the
+    feedback discussion with the "Resolve Discussion" button. This way
+    the reviewers know which feedback got handled and which didn't.
   <li>Old, stale MR may be closed, but you can reopen it if you
     still want to pursue the changes
   <li>You should periodically check to see if your MR needs to be