Improve documentation

[sashamitrovich]

This paragraph:

If you want to use the default graphSearch and graphExpand functions, which return the node and link data powering the graph, you need to install the mlpm module (if you want to create your own, you could use this mlpm_module code as a starting point or roll your own):

is not clear. How do I decide whether I want to use the default graphSearch and graphExpand functions if I, for instance, don't know what that means. Suggestion: add some explanation or a link to an explanation so that a user can make a qualified decision whether they need this.