Add deprecation advice to developer guide.

Can we have a description in the following paragraph that describes what should be placed in `<VERSION>` please? Something like:

> where you should replace `<VERSION>` with the version of Iris in which * this API is [deprecated|removed]

Deleting as appropriate; see, I knew it would be useful!

* Note that this is an example of one of those weird foibles with English that means that we can use "which" _without_ the need for a comma!