Add documentation for lots of the "undocumented" bits

In the Guile Documenta generated documentation.

knots/parallelism.scm

@@ -326,6 +326,10 @@ write directly to @code{process-channel}."
   (make-parallelism-limiter-record resource-pool)
   parallelism-limiter?
   (resource-pool     parallelism-limiter-resource-pool))
+(set-procedure-property!
+ (macro-transformer (module-ref (current-module) 'parallelism-limiter?))
+ 'documentation
+ "Return @code{#t} if OBJ is a @code{<parallelism-limiter>}.")
 
 (define* (make-parallelism-limiter limit #:key (name "unnamed"))
   "Return a parallelism limiter that allows at most LIMIT concurrent
@@ -340,6 +344,7 @@ time.  Further fibers block until a slot becomes free.
     #:name name)))
 
 (define (destroy-parallelism-limiter parallelism-limiter)
+  "Destroy PARALLELISM-LIMITER, releasing its underlying resource pool."
   (destroy-resource-pool
    (parallelism-limiter-resource-pool
     parallelism-limiter)))
@@ -354,6 +359,8 @@ currently available."
       (thunk))))
 
 (define-syntax-rule (with-parallelism-limiter parallelism-limiter exp ...)
+  "Evaluate EXP ... while holding a slot from PARALLELISM-LIMITER.
+Syntactic sugar around @code{call-with-parallelism-limiter}."
   (call-with-parallelism-limiter
       parallelism-limiter
     (lambda () exp ...)))