Add `org-crypt-decrypt-after-save-magic' function to complement the
currently existing `org-crypt-use-before-save-magic' function. Causes the
buffer to be unencrypted after it has been saved to file so that you can
continue working on it without manually running `org-decrypt-entries'
This has the downside of the buffer always being considered as modified,
but is a useful optional function.

Also updated the suggested org-crypt config in the org-manual to include
(org-crypt-decrypt-after-save-magic)