Small comment/doc updates

Author: graeme-a-stewart

src/ClusterSequence.jl

@@ -208,7 +208,9 @@ add_step_to_history!(clusterseq::ClusterSequence, parent1, parent2, jetp_index,
         clusterseq.history[parent2] = @set hist_elem.child = local_step
     end
 
-    # Get cross-referencing right
+    # Cross referencing has to be correct by construction now, these asserts
+    # can be used to check that when debugging - they do cost a bit of time
+    # which is why they are commented out for production
     # if jetp_index != Invalid
     #     @assert jetp_index >= 1
     #     @assert clusterseq.jets[jetp_index]._cluster_hist_index == local_step

src/CommonJetStructs.jl

@@ -24,7 +24,7 @@ abstract type Jet <: FourMomentum end
 """
     struct PlainJet <: FourMomentum
 
-Simple jet structure without cluster history, used as an intermediate
+Simple four momentum structure without cluster history, used as an intermediate
 type during jet merging.
 """
 struct PlainJet <: FourMomentum
@@ -73,6 +73,8 @@ Return the energy component of the four-momentum vector of `j`.
 """
 energy(j::FourMomentum) = j.E
 
+"""Alias for `energy` function"""
+const E = energy
 
 """
     p2(j::FourMomentum)
@@ -89,15 +91,15 @@ Return the squared transverse momentum of the four-momentum vector of `j`.
 """
 pt2(j::FourMomentum) = j.px^2 + j.py^2
 
+"""Alias for `pt2` function"""
+const kt2 = pt2
+
 """
     pt(j::FourMomentum)
 Return the momentum of the four-momentum vector of `j`.
 """
 pt(j::FourMomentum) = sqrt(pt2(j))
 
-const kt2 = pt2
-const E = energy
-
 """
     mass2(j::FourMomentum)