Add documentation for lots of the "undocumented" bits

In the Guile Documenta generated documentation.

knots/promise.scm

@@ -39,6 +39,10 @@
   (thunk                fibers-promise-thunk)
   (values-box           fibers-promise-values-box)
   (evaluated-condition  fibers-promise-evaluated-condition))
+(set-procedure-property!
+ (macro-transformer (module-ref (current-module) 'fibers-promise?))
+ 'documentation
+ "Return @code{#t} if OBJ is a @code{<fibers-promise>}.")
 
 (define (fibers-delay thunk)
   "Return a new fiber-aware promise that will evaluate THUNK when