lisp/org.el: Explicit the docstrings of org-priority-highest/lowest

* lisp/org.el (org-priority-highest, org-priority-lowest): Make
the docstring more explicit.

lisp/org.el

@@ -2448,8 +2448,20 @@ set a priority."
 
 (defcustom org-priority-highest ?A
   "The highest priority of TODO items.
+
 A character like ?A, ?B, etc., or a numeric value like 1, 2, etc.
-Must be smaller than `org-priority-lowest'."
+
+The default is the character ?A, which is 65 as a numeric value.
+
+If you set `org-priority-highest' to a numeric value inferior to
+65, Org assumes you want to use digits for the priority cookie.
+If you set it to >=65, Org assumes you want to use alphabetical
+characters.
+
+In both cases, the value of `org-priority-highest' must be
+smaller than `org-priority-lowest': for example, if \"A\" is the
+highest priority, it is smaller than the lowest \"C\" priority:
+65 < 67."
   :group 'org-priorities
   :type '(choice
 	  (character :tag "Character")
@@ -2458,8 +2470,20 @@ Must be smaller than `org-priority-lowest'."
 (defvaralias 'org-lowest-priority 'org-priority-lowest)
 (defcustom org-priority-lowest ?C
   "The lowest priority of TODO items.
-A character like ?A, ?B, etc., or a numeric value like 1, 2, etc.
+
-Must be higher than `org-priority-highest'."
+A character like ?C, ?B, etc., or a numeric value like 9, 8, etc.
+
+The default is the character ?C, which is 67 as a numeric value.
+
+If you set `org-priority-lowest' to a numeric value inferior to
+65, Org assumes you want to use digits for the priority cookie.
+If you set it to >=65, Org assumes you want to use alphabetical
+characters.
+
+In both cases, the value of `org-priority-lowest' must be greater
+than `org-priority-highest': for example, if \"C\" is the lowest
+priority, it is greater than the highest \"A\" priority: 67 >
+65."
   :group 'org-priorities
   :type '(choice
 	  (character :tag "Character")