[mlir][Interfaces] Tidy up the documentation for interfaces

The documentation has become a bit stale with age, and doesn't include great documentation for some newer concepts. This revision tidies up a majority of it, with some more cleanup to come in the future. The documentation for the declarative specification is also moved from OpDefinitions.md to Interfaces.md, which is a much more logical place for it to live.

Differential Revision: https://reviews.llvm.org/D92895