constify command docs

constify command docs

This makes the command "doc" parameter const.

2014-07-24  Tom Tromey  <tromey@redhat.com>

	* cli/cli-decode.c (add_cmd, add_prefix_cmd)
	(add_abbrev_prefix_cmd, add_set_or_show_cmd, add_info)
	(add_info_alias, add_com): Make "doc" const.
	(print_doc_line): Make "str" const.
	(delete_cmd): Update.
	* cli/cli-decode.h (struct cmd_list_element) <doc>: Now const.
	(print_doc_line): Update.
	* cli/cli-script.c (document_command): Update.
	* command.h (add_cmd, add_prefix_cmd, add_abbrev_prefix_cmd)
	(add_com, add_info, add_info_alias): Update.
	* guile/scm-cmd.c (cmdscm_destroyer): Update.
	* python/py-cmd.c (cmdpy_destroyer): Update.