Documenting opt.Option.filter behaviour

The documentation came out of issue/question #8.
Documenting opt.Options.Comparer

Since I just added documentation for changing filter, I also thought
about commenting on the fact that Comparer must not be changed after
creation of a database.
Documenting backward compatability of bloom filter
Merge pull request #9 from JensRantil/filter-runtime-loading

Documentation improvements of bloom filter and opt.Options