[mlir] Partially update the conversion-to-llvm document

This document was not updated after the LLVM dialect type system had been
reimplemented and was using an outdated syntax. Rewrite the part of the
document that concerns type conversion and prepare the ground for splitting it
into a document that explains how built-in types are converted and a separate
document that explains how standard types and functions are converted, which
will better correspond to the fact that built-in types do not belong to the
standard dialect.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D93486