add: ObsoleteAttribute for soon to be removed API

[linkdotnet]

Fixes #962

Basically I added some ObsoleteAttribute to the diff API that isn't around in v2 anymore.

[egil]

I don't think this is the right use of obsolete. My understanding is that you add obsolete attributes to things that have an alternative available. That wont be the case in v1, so these does not make sense.

[linkdotnet]

I don't think this is the right use of obsolete. My understanding is that you add obsolete attributes to things that have an alternative available. That wont be the case in v1, so these does not make sense.

Fair points - maybe a good documentation is also good enough. We can close that for now.