Document node parameter usage with matrix projects

The Matrix Tie Parent plugin has been deprecated for a while and the
documentation for it in JJB mentions that it's deprecated, but the
documentation doesn't call out how to use the new way of doing it.  This
patch updates the documentation to explicitly call out that the node
parameter only controls the matrix job's parent job.

Change-Id: Ie784f6d90667a0bc66ae0efd3eda1974ddbac4f2
Merge "Document node parameter usage with matrix projects"