Add examples covering anonymous macros

[toddjonker]

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          I am a little uncomfortable with "null" here because that is a value in Ion and also symbols have *undefined* text (which are the syntax of these things).  I wonder if we should use a term like "undefined" which could be encoded as not specified in the definition, a literal `null`, or even `$0` if we so choose.

Originally posted by @almann in https://github.com/amazon-ion/ion-docs/pull/205#discussion_r1176755297