Followup to D15488, which added a rule for {PHID-XXXX-nnnn} to work like monograms do, partially to make use cases like automatically composing remarkup snippets in API clients easier.

Some things that might be nice in the future (but also might not be necessary) are:

• Also link (but do not change the text of) raw PHIDs without the {...} brackets.
• Make PHIDs which correspond to embeddable objects with their own special rules (like users and projects) run the same display code that the object would, so @alincoln and {PHID-USER-alincolnphid} do the same thing.
• Maybe document that this works, although it's an extreme high-end power user feature.