Hierarchical libraries document updated

[Simon Marlow]

=20
> What's the Module field in the module header comment for?

No deep reasons.  Some shallow reasons: it looks nicer to have the
module name at the top of the module, and very naive tools which pull
out bits from the header might want to get at the module name. =20

Haddock could easily detect any inconsistencies, but it doesn't.

I'm not that strongly attached to keeping it...

Cheers,
	Simon