[Development] How to do deprecation

[Edward Welbourne]

I wrote:
>> TL;DR: there's a new [[Deprecation]] wiki page, please review.

Friedemann Kleint (1 August 2022 09:49) replied:
> It misses an important part, IMO:

> Try to get an idea of the impact of your planned deprecation by building
> all of Qt and ideally Qt Creator (producing patches fixing the
> deprecation warnings).
>
> If that turns into an huge exercise taking several days, it becomes
> clear that the deprecation is not feasible.

Fair point.  I have expanded the last paragraph of the introduction
(which touched on this, at least) - please review,

	Eddy.