Review diagnostics and develop guidelines for creating new ones

[jandubois]

We should review all existing diagnostics to develop guidelines (and adjust the diagnostics to follow our new guidelines):

• What categories should we create for diagnostics?
• Can we find a way to link categories to Preference areas?
• How do categories map to documentation pages?
• How should the wording be for passing diagnostics?
• How should the wording be for failing diagnostics?
• What should we log for passing diagnostics?
• What should we log for failing diagnostics?
• Should we log additional information at the debug level?
• How can we keep sensitive information out of the log file by default (because people may post them on Slack/Github)?

Any additional guidelines for "Fix actions"? May need to be a separate story, as the feature does not exist yet.