bug#16922: df --si -h should emit a warning

[Paul Eggert]

Pádraig Brady wrote:
> +  -h, --human-readable  print abbreviated sizes in powers of 1024 (e.g. %s)\n\
> +  -H, --si              print abbreviated sizes in powers of 1000 (e.g. %s)\n\

Thanks for looking into improving this.  Please put commas after "e.g.". 
 Also, I suggest removing "abbreviated", and simplifying the 
translator's job by simply interpolating the human.  E.g.:

-h, --human-readable  print sizes in powers of 1024 (e.g., 1023M)\n\
-H, --si              print sizes in powers of 1000 (e.g., 1.1G)\n\