Add view-doc generic

This function should return a string that explains what the view does.
This should remove the need to compile the undonerc in order to get
the explanation.

undone/main.scm

@@ -219,5 +219,5 @@
          (for-each
           (lambda (view)
             (format #t "~a~20t~a~%"
-                    (car view) (procedure-documentation (cdr view))))
+                    (car view) (view-doc (make (cdr view)))))
           view-list)))))

undone/view.scm

@@ -1,13 +1,14 @@
 (define-module (undone view)
   #:use-module (oop goops)
   #:export (<view> view-list sort-view filter-view print-item
-                   get-list-for print-view))
+                   get-list-for print-view view-doc))
 
 (define-generic sort-view)
 (define-generic filter-view)
 (define-generic print-item)
 (define-generic get-list-for)
 (define-generic print-view)
+(define-generic view-doc)
 
 (define-class <view> ()
   (todo-list #:init-value '() #:init-keyword #:todo-list))
@@ -33,3 +34,6 @@
   (for-each
    (lambda (item) (print-item v item))
    (get-list-for v)))
+
+(define-method (view-doc (v <view>))
+  "Default view, show only the basics.")