message: Clarify Primal's introduction.
diff: PRIMAL: Geometric primitives and associated computational geometry tests {#primaltop}
  ========
  -[Primal](@ref axom::primal) provides classes representing geometric shapes (such as [points](@ref axom::primal::Point), [bounding boxes](@ref axom::primal::BoundingBox), and [triangles](@ref axom::primal::BoundingBox)) and functions implementing computational geometry tests (such as intersect(), squared_distance(), orientation()).
  +[Primal](@ref axom::primal) provides classes representing geometric primitives (such as [points](@ref axom::primal::Point), [bounding boxes](@ref axom::primal::BoundingBox), and [triangles](@ref axom::primal::BoundingBox)) and functions implementing computational geometry tests (such as intersect(), squared_distance(), orientation()).
  -It also provides spatial acceleration data structures over collections of spatial objects (including [bounding volume hierarchy](@ref axom::primal::BVHTree) tree and [uniform grid](@ref axom::primal::UniformGrid) or Verlet list).
  +It also provides spatial acceleration data structures over collections of spatial objects, including the [bounding volume hierarchy](@ref axom::primal::BVHTree) tree and the [uniform grid](@ref axom::primal::UniformGrid) (also known as cell list).
  The [introduction to Primal](../../../sphinx/primal_docs/html/index.html)
  covers these concepts in more detail.
  