org-capture.el: improve docstring for `org-capture-before-finalize-hook'.

* org-capture.el (org-capture-before-finalize-hook): Docstring
improvement: mention that the buffer is widened when this hook
is run.

Thanks to Paul for a suggestion about this.

lisp/org-capture.el

@@ -333,7 +333,8 @@ calendar                |  %:type %:date"
 
 (defcustom org-capture-before-finalize-hook nil
   "Hook that is run right before a capture process is finalized.
-The capture buffer is still current when this hook runs."
+The capture buffer is still current when this hook runs and it is
+widened to the entire buffer."
   :group 'org-capture
   :type 'hook)