error: document all entry points provided

error: document all entry points provided

Based on a report by Mohammad Akhlaghi.

* doc/glibc-functions/error_print_progname.texi
(error_print_progname): Mention the error module.
* doc/glibc-functions/error_at_line.texi (error_at_line):
Likewise.
* doc/glibc-functions/error_message_count.texi
(error_message_count): Likewise.
* doc/glibc-functions/error_one_per_line.texi
(error_one_per_line): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>