Specify limitation of nodePartition in Placement Constraints document

PlacementConstraints supports affinity to only single partition currently in SingleConstraintAppPlacementAllocator.

if (values.size() > 1) { 
throwExceptionWithMetaInfo( "Inside one targetExpression, we only support" + " affinity to at most one node partition now"); 
}

But in the docs we don't mention this limitation.