* doc/org.texi (Setting up the staging area): Document use of crypt password.
* lisp/org-mobile.el (org-mobile-encryption-password): Improve docstring.
(org-mobile-encryption-password-session): New variable.
(org-mobile-encryption-password): New function.
(org-mobile-check-setup):
(org-mobile-encrypt-file):
(org-mobile-decrypt-file): Use the new function.