[MLIR] Updates around MemRef Normalization

The documentation for the NormalizeMemRefs pass and the associated MemRefsNormalizable
traits was confusing and not on the website.  This update clarifies the language
around the difference between a MemRef Type, an operation that accesses the value of
MemRef Type, and better documents the limitations of the current implementation.
This patch also includes some basic debugging information for the pass so people
might have a chance of figuring out why it doesn't work on their code.

Differential Revision: https://reviews.llvm.org/D88532