I think it's worth pointing out that while an advanced degree in some random field is certainly not the same thing as real-world technical writing expertise... an advanced degree in technical writing should include a great deal of attention to technical writing, and is of substantial value.

It's rare to find a technical writer who has a master's in technical writing itself, of course, but they do exist. Still, your typical tech writer has a degree in something like communications or English, and should be judged more on their writing experience and the quality of their portfolio. (And a technical writer without a portfolio isn't worth hiring at all.)

One of the primary capabilities that a good technical writer brings to a writing project, is the ability to assess the technical savvy of a particular audience, and tailor the language to best fit their needs.

A single catch-all rule about how detailed or specific a given document should be is foolish. Audiences differ; writing needs to be adapted to suit the need.

It is really no different than designing a user interface; something you'd design for a children's counting game is very different than a console intended for system administrators to review the performance of enterprise software deployments. And both are different than the UI intended for an ATM or an airport check-in kiosk.

A technical writer who is worth his or her salt, begins not with the material but with the audience; their needs, their level of technical sophistication, their primary languages, the organizational schemas they are likely to be familiar with, and so on.

The second step of course is to understand the material; and the third, is to act as a translator, converting the material into the language of the audience.

I came in here specifically to say what this poster just said. You can hire a technical writer on a contract basis to handle just this particular issue... or, take a look and see where else a good technical writer could make a contribution and consider hiring a staff writer.

That's not to say subby shouldn't attempt to document his own procedures. Just, that a technical writer can take what is produced and convert it into something that is accurate, complete, written in the language that the audience finds most accessible, and can recommend organizational schemas that are maximally effective for your particular audience.