Avoid memory leaks on struct cmd_list_element.doc field.

	Avoid memory leaks on struct cmd_list_element.doc field.
	* cli/cli-decode.c (add_alias_cmd): Make a copy of doc field
	if flags contains DOC_ALLOCATED.
	(add_setshow_cmd_full): Add DOC_ALLOCATED to set and show
	flags.
	(delete_cmd): Handle DOC_ALLOCATED flag.
	* cli/cli-decode.h (DOC_ALLOCATED): New macro for use
	in flags filed of struct cmd_list_element.
	(struct cmd_list_element): Document new flag item.