Edit to tech writer onboarding process and stage/group assignments

Why is this change being made?

Due to decisions made when resolving gitlab-org/gitlab-design#825 (closed):

• Clarify that more than one group can be assigned at once to a trainee, and the coach can merge unless it's more appropriate to approve and reassign, in keeping with a decision made and implemented for recent team reallocation.
• Add a link to the planned assignment changes.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what