GitHub import: improve documentation

What does this MR do and why?

Implementation for GitHub import: improve documentation (#392754 - closed)

Improves documentation based on the comment here

GitHub import-related documentation has some parts that are not relevant anymore and where not removed on time of code changes.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

doc/user/project/import/github.md

@@ -81,7 +81,8 @@ Before you begin, ensure that any GitHub user you want to map to a GitLab user h
 
 If you are importing to GitLab.com, you can alternatively import GitHub repositories using a [personal access token](#use-a-github-token).
 
-If GitHub user public email doesn't match any GitLab user email, the activity is associated with the user account that is performing the import.
+If a GitHub user's public email address doesn't match any GitLab user email address, the user's activity is associated with the user account that is
+performing the import.
 
 NOTE:
 If you are using a self-managed GitLab instance or if you are importing from GitHub Enterprise, this process requires that you have configured