Explicitly require public visibility of emails

The GitHub import performs a REST API call to fetch a user's email to
associate their content to their GitLab account.
The REST API of GitHub will show a `null` email address field if the
primary email address is not marked to be publicly visible in the
account's profile settings page.

Therefore the doc must (more) explicitly state the need for a publicly
visible email address on the GitHub end prior to the import.

doc/user/project/import/github.md

@@ -59,9 +59,9 @@ For this association to succeed, each GitHub author and assignee in the reposito
 must meet one of the following conditions prior to the import:
 
 - Have previously logged in to a GitLab account using the GitHub icon.
-- Have a GitHub account with a
-  [primary email address](https://docs.github.com/en/free-pro-team@latest/github/setting-up-and-managing-your-github-user-account/setting-your-commit-email-address)
-  that matches their GitLab account's email address.
+- Have a GitHub account with a publicly visible
+  [primary email address](https://docs.github.com/en/free-pro-team@latest/rest/reference/users#get-a-user)
+  on their profile that matches their GitLab account's email address.
 
 If a user referenced in the project is not found in GitLab's database, the project creator (typically the user
 that initiated the import process) is set as the author/assignee, but a note on the issue mentioning the original
@@ -89,7 +89,7 @@ Before you begin, ensure that any GitHub users who you want to map to GitLab use
 
 - A GitLab account that has logged in using the GitHub icon
   \- or -
-- A GitLab account with an email address that matches the [public email address](https://docs.github.com/en/free-pro-team@latest/github/setting-up-and-managing-your-github-user-account/setting-your-commit-email-address) of the GitHub user
+- A GitLab account with an email address that matches the [publicly visible email address](https://docs.github.com/en/free-pro-team@latest/rest/reference/users#get-a-user) in the profile of the GitHub user
 
 User-matching attempts occur in that order, and if a user is not identified either way, the activity is associated with
 the user account that is performing the import.