Fix instance rules for functional dependencies

Fix instance rules for functional dependencies

GHC 6.4 implements a rather relaxed version of the Coverage Condition
which is actually too relaxed: the compiler can get into an infinite loop
as a result.

This commit fixes the problem (see Note [Coverage condition] in FunDeps.lhs)
and documents the change.

I also took the opportunity to add documentation about functional dependencies,
taken from the Hugs manual with kind permission of Mark Jones