Improve and clarify `config.status' usage documentation.

Improve and clarify `config.status' usage documentation.

* doc/autoconf.texi (config.status Invocation): Fix markup in
synopsis.  Use `tag' instead of `file' notation for the
non-option arguments, to be consistent with the documentation
of the AC_CONFIG_* macros.
* lib/autoconf/status.m4 (_AC_OUTPUT_CONFIG_STATUS): Likewise,
use `tag' notation for non-option arguments.
Report by John Calcote.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>