[PATCH] manual: Document %m instead of %a for formatted input.

[Roland McGrath]

> 	* manual/stdio.texi (Dynamic String Input): Recommend %m modifier instead of %a.

This line is too long.

> +A POSIX.1-2008 extension to formatted input lets you safely

It doesn't make sense to talk about "a POSIX.1-2008 extension", even if
that were accurate.  (Actually it's an ISO C-1990 feature.)  Something in
one of the core standards we implement is not "an extension".  The manual
also needs to continue to document the GNU %a extension, and explain the
effect of -std mode.  %a is also mentioned in @node String Input Conversions
that needs to be updated too.