Clarify valid category names for release post items

Why is this change being made?

With the linting of the release post items, the categories section of the release post item is being checked against the ../data/categories.yml file. This change adds explanation that the name: field is what is needed in the release post item template.

I struggled with this, so am making the change in hopes of preventing others from having similar issues.

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