[MLton] Compile time options bug

[Stephen Weeks]

> The documentation (http://mlton.org/CompileTimeOptions) seems to
> imply that the following command line options are legal, yet mlton
> generates an error message (see below).
...
> $ mlton -target self -verbose 1 -stop tc -disable-ann 'forceUsed, warnUnused'
> sources_mlton.cm 
> invalid -disable-ann flag: forceUsed, warnUnused

Yeah, I guess the documentation for -disable-ann (as well as
-default-ann) has been out of date since the release.  Comma separated
args aren't allowed.  I've update the docs.