[Contributing] tweaks to the contribution chapter

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 

Some tweaks for the changes made in #4223.

@javiereguiluz Sorry,  didn't find the time to review it in the first run.
the doc you linked talks about criteria for the contributions on the profile page. for repo contributors the rules are explained at https://help.github.com/articles/i-don-t-see-myself-in-the-repository-contributors-graph

the only case impacting us here is the case of email addresses not being linked to an account (and so ignored by github when building the list) or several addresses for the same user. The criteria about contributing to the default branch is not an issue given that other branches are merged in master regularly