[mlir][quant] Move comments to TableGen statements

The `quant` dialect documentation currently mostly empty (https://mlir.llvm.org/docs/Dialects/QuantDialect/).
This patch moves the comments to TableGen statements.
By looking at the generated `QuantDialect.md`, I've confirmed that each change in this patch will end up in the documentation.

The only thing that I wasn't able to document was the `UniformQuantizedType`.
I haven't found a way to document a `DialectType` since most appear to be private in other dialects; suggestions are welcome.

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D151649