bug#50068: Curried definitions lose docstrings

When a docstring is used in the body of a curried definition,
it ends up on the result of the application of the resulting
curried function, not on the function itself. Example:

(use-modules (ice-9 curried-definitions))

(define ((curried a) b)
   "Docstring of curried"
   'whatever)

(procedure-documentation curried)
=> #f

(procedure-documentation (curried 'whatnot))
=> "Docstring of curried"


This is apparently because Guile translates

(define ((curried a) b)
   ...)

to

(define (curried a)
   (lambda (b)
     ...))

which puts the docstring in the body
of lambda.

Tested with Guile 2.2 and 3.0.5.