Update truncation with illustration/text modeled on webauthn

[aphillips]

See: w3c/webauthn#1527

webauthn 2.0 has this section: https://w3c.github.io/webauthn/#sctn-strings

This contains examples and illustrations that might be useful in helping other spec authors describe different sorts of truncation and which might be helpful in normatively specifying same. This issue is basically a task for someone to review our text and incorporate any useful bits, with an eye towards allowing webauthn and other specs to dispense with any prose and just reference our materials.

[aphillips]

Thinking aloud: should we consider splitting off a Note or article on string truncation?