Improve `org-insert-drawer' and related documentation.

Improve `org-insert-drawer' and related documentation.

* org.el (org-insert-property-drawer): Not an interactive
command anymore.
(org-insert-drawer): With a prefix argument, insert a property
drawer.  Check for headline within the region before inserting
the drawer.  Don't include special drawers in the completion
table.
(org-mode-map): New keybinding `C-c C-x d' for
`org-insert-drawer'.

* org.texi (Drawers): How to insert/complete drawers.

Thanks to Nicolas Goaziou for the discussion and the patch.