Discuss use of docs-only label and then document or remove it

Issue Description

As part of #84 (closed) we discussed (and removed) several labels that were deemed to be deprecated. As part of those discussions, we talked about the docs-only label, and there was no clear consensus on whether we should keep the label or remove it.

I'd like for this issue to be a discussion area on this label. Questions to answer can include: what do we use it for right now, is that use across the team, what's the utility of reporting this information, who uses this information?

If we feel that we should continue to use this label, we should use this issue to include it in our list of labels in use at https://about.gitlab.com/handbook/engineering/ux/technical-writing/workflow/#labels and also ensure it's in our template descriptions and team workflows to encourage the label's proper use. If we decide to remove the label, then this issue can also handle that.

Mentioning @gl-docsteam for awareness.