[llvm-mca][docs] Add documentation for the statistic outputs from mca. NFC

Summary: The original text was lifted from the MCA README.  I re-ran the dot-product example and updated the output seen in the docs.  I also added a few paragraphs discussing the instruction issued and retired histograms, as well as discussing the register file stats.

Reviewers: andreadb, RKSimon, courbet, gbedwell, filcab

Reviewed By: andreadb

Subscribers: tschuett

Differential Revision: https://reviews.llvm.org/D49614

llvm-svn: 337648