Update documentation of dseries

Since dseries have been substantially rewritten since 4.5, the documentation in the reference needs to be updated.

added documentation dseries labels

See 12ee697a, 615778da and 91a4c1c7.

@stepan-a Can this issue be closed, or is there something still to be done?

According to @stepan-a:

Almost everything is done, and there should be no wrong information anymore.

A few things may still be missing in the doc and need to be checked (e.g. how to retrieve the years and subperiods of a dates object, to reconstruct the time member as it was before).

changed milestone to %6.x

Added entry for the undocumented dplot Matlab command in f5a5ebb9. Will document the from Matlab command (hidden since ages) later, and I leave this for another ticket.

closed