Mismatch between example documentation and actual practice

As far as I can tell, the documentation for how the legacy_params function should be written is quite limited, and canonical source of info is its implementation and related comments in useless.c.

There, the main comment says (among other things):

// Upgrading from an older version than the previous one should always
// go through all in between versions (unless there was a bug) so that
// the end result will always be the same.

However, that does not seem to be the strategy adopted by many/most modules, which instead directly from various old versions to the newest.

So, is this actually recommended?

• If not, maybe that comment should be dropped.
• If yes, maybe that should be surfaced in a more visible place, such as iop/iop_api.h, and/or https://github.com/darktable-org/darktable/blob/master/dev-doc/IOP_Module_API.md

Start by reading the legacy_params comment and implementation in useless.c, then compare that guidance with the legacy_params practices used by modules. Review iop/iop_api.h and dev-doc/IOP_Module_API.md as possible documentation locations. Done means the recommended upgrade strategy is resolved and the relevant comment or documentation consistently communicates it.