org-manual: Clarify nil value of ~org-crypt-key~

* doc/org-manual.org (Org Crypt): Explain that nil value of
~org-crypt-key~ bypasses the per-heading properties.

Reported-by: Ignacio Casso <ignaciocasso@hotmail.com>
Link: https://orgmode.org/list/PAXPR06MB7760DA356C7C27045BEB64F6C6449@PAXPR06MB7760.eurprd06.prod.outlook.com

doc/org-manual.org

@@ -20668,8 +20668,9 @@ Here is a suggestion for Org Crypt settings in Emacs init file:
 (setq org-tags-exclude-from-inheritance '("crypt"))
 
 (setq org-crypt-key nil)
-;; GPG key to use for encryption
-;; Either the Key ID or set to nil to use symmetric encryption.
+;; GPG key to use for encryption.
+;; nil means  use symmetric encryption unconditionally.
+;; "" means use symmetric encryption unless heading sets CRYPTKEY property.
 
 (setq auto-save-default nil)
 ;; Auto-saving does not cooperate with org-crypt.el: so you need to
@@ -20691,6 +20692,10 @@ specifying the respective key as property =CRYPTKEY=, e.g.:
   :END:
 #+end_example
 
+Note that =CRYPTKEY= property is only effective when ~org-crypt-key~
+is set to non-nil.  ~nil~ value of ~org-crypt-key~ makes Org use
+symmetric encryption unconditionally.
+
 Excluding the =crypt= tag from inheritance prevents already encrypted
 text from being encrypted again.