Improving documentation for Comparer interface

By moving the orphaned "Advanced function" documentation block into the
individual function documentation blocks, it will show up in
http://godoc.org/github.com/syndtr/goleveldb/leveldb/comparer#Comparer.
I was frustrated with the fact that godoc.org didn't explain why these
functions really existed. Even reading the documentation code took me
a while before I saw the comment block above the two functions.

Hopefully this commit will make it easier for the next person after me.