[pitay]

I am assuming you are talking about database indexes. The documentation for the database that I have most used actually specified what the various supported types were good for. It made choosing the correct type of index much easier. The documentation may be sufficient for determining which index types will perform well for the expected queries, even if knowing detailed information about data structures is nice to know. Having documentation which describes the data structures well enough is really nice though, and should be encouraged a lot.