Add documentation for make-guardian.

* module/ice-9/guardians.scm (make-guardian): Add documentation from old
C version.

module/ice-9/guardians.scm

@@ -88,6 +88,8 @@
   (values push! pop!))
 
 (define (make-guardian)
+  "Create a new guardian.  A guardian protects a set of objects from
+garbage collection, allowing a program to apply cleanup or other"
   (define-values (push! pop!) (make-atomic-fifo))
   (define (guard! obj)
     (when (heap-object? obj)