docs: Add list of nodes for 'trace add'

docs:  Add list of nodes for 'trace add'

(provided by Andrew/Damjan: git grep -h -B20 VLIB_NODE_FLAG_TRACE_SUPPORTED | grep VLIB_REGISTER_NODE | awk -F [\(\),] '{print $2 }' | sed 's/_/-/g' | sort |sed 's/-node//' )

Type: docs

Change-Id: I4a58fe5ed6bb3598551482ea1c5f16e502cbd6a7
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>