thunk.el: Document that thunk-force == funcall?

[Michael Heerdegen]

Hello,

`thunk-force' is equivalent to `funcall' - thunks are functions.  I
wonder if we could/should officially document that fact?

The background: because thunks are functions, one is allowed (and it is
useful) to directly pass them to higher order functions e.g. as a test
predicate, or bind them using `cl-flet', or `cl-letf' to a
`symbol-function' place.  Currently, the official solution would require
to use a lambda wrapper.

I have encountered this requirement several times (and using `thunk-let'
was not always the solution) - thus my question.

TIA,

Michael.