Mention graph merging in merge/union description

[rubensworks]

Currently, the description of this method is the following:

Returns a new Dataset that is a concatenation of this dataset and the quads given as an argument.

However, "concatenation" may be an oversimplification. If any of the datasets contain equal blank nodes, then the blank nodes should be standardized apart according to the RDF spec.

It may be good to link to the section on graph merging in the RDF spec.

Originally mentioned in #19 (comment).