Add guidance around Undo

[cassidyjames]

Something @matthewpaulthomas mentioned is that we're not super clear around how to implement undo actions. We should probably:

• Talk about keyboard shortcuts somewhere (covering Ctrl+Z)
• Update the existing Undo section to mention toasts
• Mention contextual undo/redo actions, like in the context menu in Code

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[matthewpaulthomas]

Undo was merely an example of something that has a fairly well-known position in menu bars, but no obviously consistent access point without them. A few other examples, similarly unspecified in the HIG: Redo, Repeat, Paste As, Export, Print, Zoom In/Out, Help, and Find/Change when Change is more complex than just Replace (and therefore perhaps too space-consuming for Find’s “revealer”, though I can’t be sure because “revealer” itself isn’t defined in the HIG).

When writing guidelines specifically for Undo, though, a few more things to cover:

• In what kinds of apps, windows, or modes should Undo+Redo have dedicated buttons?
• When and how should I indicate that something isn’t undoable?
• What should the scope of an undo stack be? (For example, if in a photo manager I make one tweak to photo A then one to B then one more to A, then choose Undo twice, which two actions should be undone?)
• How should I decide a coalescing policy? (For example, if I type 150 words then choose Undo once, how much should become un-typed?)