[MLIR][Bufferization] Improve documentation of introduction and `summary`s

This patch suggests to add a sentence in the introduction which clarifies
the aim of the dialect in one sentence. I hope that I understood it correctly.

For the `summary` fields, I have scrolled through the Bufferization page
(<https://mlir.llvm.org/docs/Bufferization>) and added missing summaries
or standardized existing ones to make the summaries easier to read.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D151829