Add documentation for lots of the "undocumented" bits

In the Guile Documenta generated documentation.

knots/backtraces.scm

@@ -41,14 +41,20 @@
 
 (define make-knots-exception
   (record-constructor &knots-exception))
+(set-procedure-property! make-knots-exception 'documentation
+  "Construct a @code{&knots-exception} with the given stack.")
 
 (define knots-exception?
   (exception-predicate &knots-exception))
+(set-procedure-property! knots-exception? 'documentation
+  "Return @code{#t} if OBJ is a @code{&knots-exception}.")
 
 (define knots-exception-stack
   (exception-accessor
    &knots-exception
    (record-accessor &knots-exception 'stack)))
+(set-procedure-property! knots-exception-stack 'documentation
+  "Return the stack from a @code{&knots-exception}.")
 
 (define (backtrace-debug-mode?)
   (let ((val (getenv "KNOTS_BACKTRACE_DEBUG")))
@@ -222,6 +228,34 @@
 (define* (print-backtrace-and-exception/knots
           exn
           #:key (port (current-error-port)))
+  "Print the backtrace and exception information from EXN to PORT. This
+procedure captures the stack, so should be run before the stack is
+unwound, so using @code{with-exception-handler} without
+@code{#:unwind? #t}, the exception may need to then be re-raised and
+handled in an outer exception handler.
+
+@example
+(with-exception-handler
+    (lambda (exn)
+      ;; Recover from the exception
+      #f)
+  (lambda ()
+    (with-exception-handler
+        (lambda (exn)
+          (print-backtrace-and-exception/knots exn)
+          (raise-exception exn))
+      (lambda ()
+        (do-things))))
+  #:unwind? #t)
+@end example
+
+It's important to use @code{print-backtrace-and-exception/knots} for
+displaying backtraces involving functionality from Guile Knots, since
+the stack involved is potentially split across several fibers. The
+stacks involved are attached to the exception, and this procedure
+extracts this information out and assembles a backtrace including all
+the code involved.
+"
   (define (get-string out stack)
     (let* ((stack-vector (stack->vector stack))
            (knots-stack-vectors