SubmittingPatches: update extra tags list

Add items with at least 100 uses in the past three years:
- Co-authored-by
- Helped-by
- Mentored-by
- Suggested-by

git log --since=3.years|
  perl -ne 'next unless /^\s+[A-Z][a-z]+-\S+:/;s/^\s+//;s/:.*/:/;print'|
  sort|uniq -c|sort -n|grep '[0-9][0-9] '
  14 Based-on-patch-by:
  14 Original-patch-by:
  17 Tested-by:
 100 Suggested-by:
 121 Co-authored-by:
 163 Mentored-by:
 274 Reported-by:
 290 Acked-by:
 450 Helped-by:
 602 Reviewed-by:
14111 Signed-off-by:

Signed-off-by: Josh Soref <jsoref@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -355,6 +355,14 @@ If you like, you can put extra tags at the end:
   patch after a detailed analysis.
 . `Tested-by:` is used to indicate that the person applied the patch
   and found it to have the desired effect.
+. `Co-authored-by:` is used to indicate that people exchanged drafts
+   of a patch before submitting it.
+. `Helped-by:` is used to credit someone who suggested ideas for
+  changes without providing the precise changes in patch form.
+. `Mentored-by:` is used to credit someone with helping develop a
+  patch as part of a mentorship program (e.g., GSoC or Outreachy).
+. `Suggested-by:` is used to credit someone with suggesting the idea
+  for a patch.
 
 While you can also create your own trailer if the situation warrants it, we
 encourage you to instead use one of the common trailers in this project