[m-dev.] diff: add `--trace' option for ml

[Warwick Harvey]

Fergus wrote:
> Index: doc/user_guide.texi
[...]
> + at example
> +C2INITFLAGS = --trace
> +MLFLAGS     = --trace
> +MCFLAGS-foo = --trace deep
> +MCFLAGS-bar = --trace shalow
> + at end example

s/shalow/shallow/

Warwick