Documentation for DaphneIR operations

In GitLab by @pdamme on Apr 21, 2021, 13:08

The definitions of the *DaphneIR* operations (in `src/ir/daphneir/DaphneOps.td`) still lack documentation. Each operation should have at least a short summary, where necessary also a more detailed description. The `summary` and `description` fields of `tablegen`'s `Op`-class can be used for that.

This is important because it can be the central place documenting what each operation is supposed to do.