In the upper-right corner of the "new editor window" (the undocked document) there should be a small circle containing a downward-pointing triangle. Click that triangle and select one of the two options that start with the word Dock -- one docks just that document, one docks all the undocked documents.
If you click that same triangle for a docked document, it will let you undock the Editor or individual documents into separate windows. You can also click and drag a document's tab out of the Editor to undock it, which is probably how the first undocked document became undocked.
