bug#5154: 23.1.50; mml-attach-file: say how to attach several files

Both (info "(message) MIME") and the docstring of:
C-c C-a runs the command mml-attach-file,
should mention how to attach several files all at once.

e.g., one wants to attach
DSCN2764.JPG DSCN2765.JPG DSCN2766.JPG DSCN2769.JPG DSCN2771.JPG DSCN2772.JPG
all at once and not one at a time.

Or say it is not possible, and to use another MUA instead.