Improve MI -dprintf-insert documentation

Improve MI -dprintf-insert documentation

I found the documentation for -dprintf-insert a bit unclear.  It
didn't mention the possibility of multiple arguments, and I also
noticed that it implied that the format parameter is optional, which
it is not.

While looking into this I also noticed a few comments in the
implementation that could also be improved.

Then, I noticed a repeated call to strlen in a loop condition, so I
fixed this up as well.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>