Tech writing handbook - update dev guidelines reviews

Why is this change being made?

Clarifies that dev guidelines related to specific stages/groups should be reviewed by TWs assigned to the respective stage/group and the TW assigned to dev guidelines cc/ed for their information.

Slack thread here: https://gitlab.slack.com/archives/CCB575BGT/p1580403160356700

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