[DOCUMENTATION] Convenient code blocks and naming

[LdDl]

What is your docs question about? Ask it
Well, there are many code with some unclear priority queue stuff, such as:

...distance.distance = vertex.distance.distance + cost
...distance.contractionID = contractionID 
...distance.sourceID = sourceID

What do you suggest?

1. Refactor code to make in clear for developer.
   E.g. merge relaxEdges() and dijkstra() function in one (something like dijkstra_witness)
2. Make some kind of wiki-code in comments' sections
   E.g. explain what this code does

// checkID Checks if both source's and target's contraction ID are not equal
func (graph *Graph) checkID(source, target int64) bool {
	s := graph.Vertices[source].distance
	t := graph.Vertices[target].distance
	return s.contractionID != t.contractionID || s.sourceID != t.sourceID
}

Additional context
nope