groupdb:backend = tdb can be used to use tdb for groupmaping database. All new options should be documented so that our users have a change to know that they exist and how to use them.
Usually, parametric options are not documented in the manpage (there are exceptions, I know ;-). I guess these are "hidden" options for special tuning. So why is it a parametric option?
let me correc this: parametric options have often been undocumented in the past, mainly because writing documentation is thankless work. But all parameters should be documented because every parameter has been introduced for some reason, to fix something or whatever. If the problem or that "whatever" isn't documented our users are not able to use it and thus they are not able to solve their configuration problems. This is why we need to consider missing documentation to be a bug. In case the option is "dangerous" or "discouraged to use" then this can be written in the documentation, too.