Make it match the guidelines. Review what symbols don't adhere, and rename them. Ref: #14268 (comment)

Couple thoughts:

• If this is a regular expression, should we be assuring ^ so that we'd not inadvertently ignore the token definitely_not_unstable (ignoring the fact this would, for other reasons, not be allowed)
• The guidelines clearly prescribe the double-underscore __ prefix. I don't think we want the tolerance here. I want to see clear and loudly in the produced documentation if we're not naming these correctly.