Document the use of `slurp` with `printf`
Document the use of `slurp` with `printf`

A recent IM question, and subsequent discussion, resulted in @hhanche
commenting that if you need to explicitly move the output of `printf`
from the byte to the value stream you should use `slurp` rather than
`put` or `from-lines`. Include that insight in the `printf`
documentation.

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