Clarify transferring project, specify permissions

What does this MR do?

Documentation fix. Clarify wording about Transferring project to an existing namespace.
Also fixes an error : I believe you can transfer a project into a group where you have master access, you don't have to be owner as the doc currently stated.

Why was this MR needed?

Current documentation was ambiguous. What does "a group you manage" mean, EXACTLY?

Also there was a documentation mistake.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary -- does not seem relevant
• Documentation created/updated ¯\_(ツ)_/¯ Too Long, Didn't Read
• Branch has no merge conflicts with master (if it does - rebase it please)