docs/howto: reuse a comprehension's logic in multiple places

[jpluscplusm]

A user asked:

Is there any way to reuse a list comprehension across similar objects?

foo: [...#Thing]
bar: [...#Thing]

baz: [ for x in foo { complex logic } }
qux: [ for x in bar { identical logic to foo } }

They were pointed towards the Cuetorials "function pattern" page (https://cuetorials.com/patterns/functions/), which resolved their issue

Separately from having coverage of more general content such as that page, we should address the specific question in a howto.