[feature] Document hover

[Freed-Wu]

Comes from liuchengxu/graphviz.vim#12:

Can it provide document hover? The document can come from here. And autocompletion can also have this document.

Currently:

Expected: Display some document about items.

[Freed-Wu]

If we want to add some documents, we must change some variables from array to object. Do you agree this change?