2021: Update for or-patterns changes

[ehuss]

The metavariables section needs to be updated for the 2021 or-patterns changes. pat needs to be changed to [_Pattern_], and a separate callout section for older behavior needs to be added. See the edition guide chapter for more details.

(I think that's all that needs to be done, but I am unsure. I'm also unsure on how to integrate this change. Typically we use "Edition Differences" callout blocks, but that seems like overkill here, but it seems strange to be inconsistent.)