Revise wording on label indexing

[krlawrence]

Since the original manuscript was written, it has become more common for graph databases to index labels. It's probably a good time to revise the wording in section "3.9. Working with labels" to reflect that.

Suggested changes: Current text

As useful as labels are, in larger graph deployments when indexing technology such as Solr or Elasticsearch is often used to speed up traversing the graph, vertex labels typically do not get indexed. Therefore, it is currently recommended that an actual vertex property that can be indexed is used when walking a graph rather than relying on the vertex label. This is especially important when working with large graphs where performance can become an issue.

Possible new text

As useful as labels are, not all graph database engines provide support for indexing them. You should check to see if the graph database technology you are using allows for labels to be indexed. If that is not the case, it is recommended to use a vertex or edge property, that can be indexed, as a surrogate for the label. This can then be used in graph queries rather than relying on the vertex label. This is especially important when working with large graphs where performance can become an issue if the items you are looking for are not backed by an index.