Commit 911e7020770 added a variety of new support routines to index AMs. For example, it added a support function 5 to btree (see BTOPTIONS_PROC), but didn't document this alongside the other support functions in btree.sgml.
It looks like the new support functions are fundamentally different to the existing ones in that they exist only as a way of supplying parameters to other support functions. The idea was to preserve compatibility with the old support function signatures. Even still, I think that the new support functions should get some mention alongside the older support functions. I also wonder whether or not xindex.sgml needs to be updated to account for opclass parameters. -- Peter Geoghegan
