cfg-buildsteps.rst: Move description-attributes to BuildStep

This commit moves the description, descriptionDone, and
descriptionSuffix attributes to the BuildStep part of the document to
reflect the fact that they are moved to the BuildStep class in the
code.
cls-buildsteps.py: Adds info for description{,Done,Suffix}
relnotes/index.rst: Add info about moving description attributes

description, descriptionDone and descriptionSuffix have been moved
from ShellCommand to its superclass BuildStep.