[MLton] Implementing warnExnMatch

[Stephen Weeks]

> What is the policy for documenting features that have not yet appeared
> in a public release? 

Before the wiki, we used to document new stuff in the HEAD CVS
revision.  When we switched to the wiki, which is a lame version
control system, we didn't really spell out the policy.  But since two
versions of MLton (the last stable release and the CVS head) share the
wiki for their documentation, the only reasonable thing seems to be to
indicate with each feature if it only belongs to one version or the
other.

> Should warnExnMatch already be mentioned on the Wiki
> (http://mlton.org/MLBasisAnnotations)?
> 
> A brief note could look something like this:
> 
> <--->
> {{{warnExnMatch {true|false} }}}
>   Report nonexhaustive matches that arise solely from unmatched exceptions.
>   (Since 20050727.)
> <--->

Looks fine.  But we should wait until things shake out a little more.