mem-ruby: MESI_Three_level HTML reference generation fix

mem-ruby: MESI_Three_level HTML reference generation fix

The SLICC HTML generator does not work without the 'desc' property of
the STATES and EVENTS found in the protocol state machine source files.
This adds the 'desc' property in MESI_Three_Level to declarations where
it was missing and cleans up the text of some existing ones.

Issue-on: https://gem5.atlassian.net/browse/GEM5-357

Change-Id: I2d0f8e11889554063fed798e724217963d4a74de
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24256
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>