BookInfoManager: Actually close the document after extraction

DocumentRegistry just decreases a ref, it doesn't close anything.

Plug the same Document leak in a few other places, and document this.

frontend/document/document.lua

@@ -91,6 +91,7 @@ function Document:unlock(password)
 end
 
 -- this might be overridden by a document implementation
+-- (in which case, do make sure it calls this one, too, to avoid refcounting mismatches in DocumentRegistry!)
 function Document:close()
     local DocumentRegistry = require("document/documentregistry")
     if self.is_open then