Fix documentation (or design) of Free Monad

[samthebest]

Having played around with ScanamoOps I learnt that we cannot compare if two cats Free Monads are equal.

It seems it boils down to FlatMapped "case class" having a Function in it https://github.com/typelevel/cats/blob/main/free/src/main/scala/cats/free/Free.scala

That's fine, but the documentation says:

represent stateful computations as data, and run them

https://typelevel.org/cats/datatypes/freemonad.html

Please fix the documentation to say:

represent stateful computations as opaque function references, and run them

cc @stacycurl @davegurnell