Even knowing that this feature exists I don't know that I could have found it within a reasonable amount of time in its present location. A few cross-references from elsewhere (probably at least the functions part of the documentation) would make learning about the capability a lot easier.
Well, it's referenced right from the "Indexes" part of the documentation (right at the beginning of "Index Types"):
While I may have an understanding of what operator classes and families are when I am in my SQL thinking mode those terms don't really come to mind. Maybe part of the problem is that SQL doesn't have indexes and so my formal education never covered them. I learned how to use "CREATE INDEX" to meet most common needs but the fact that I'm getting a b-tree family index is well hidden.
While I'm all for learning the theory a more prescriptive approach (do this to get an index that will allow prefix LIKEs to use it - see this section for detail) to the topic would be welcome. Tell the user how to use an index when they are learning about the feature that they care about - LIKE - not require them to learn all about indexes and later realize/remember that one particular incantation will solve the LIKE problem.