[PATCH] manual: Document the standardized scanf flag, "m". [BZ #16376]

[Paul Eggert]

On 02/07/2018 12:46 AM, Andreas Schwab wrote:
> IMHO the "a" modifier should be marked as deprecated, since it conflicts
> with the %a format.  I think it would be enough to mention it in a
> paragraph, but nowhere else.

Agreed. Also, I suggest modifying the NEWS entry for glibc 1.06 that 
talks about the newly-introduced "a" modifier, so that it briefly 
mentions that the modifier is deprecated after glibc 2.28.