doc: additional guidance about amending commits

In my first contribution, I got the amending guidance
wrongly and amended my commit to attend some requested
changes. So I added some more lines in the docs to help the
further contributors to don't make the same mistake that I did

doc/guides/contributing/pull-requests.md

@@ -318,6 +318,8 @@ $ git commit --amend
 $ git push --force-with-lease origin my-branch
 ```
 
+Amend your last commit just to fix some minor mistake you may have noticed before any reviewing, though. To make the reviewing process simpler, when some changes are requested it's preferable to add a new commit.
+
 There are a number of more advanced mechanisms for managing commits using
 `git rebase` that can be used, but are beyond the scope of this guide.