[DOC] Mention that DeformConv2d now supports modulation

[fmassa]

📚 Documentation

With #2791, DeformConv2d now supports modulation, also known as DeformConv v2 from https://arxiv.org/abs/1811.11168

It might be good to cite this paper in the documentation of DeformConv2d and mention DeformConv v2 as well.

[fmassa]

Fixed by #3000