Double clicking with the left mouse button on a document in read mode will put it into edit mode, if you have edit rights.
Double clicking on a word when a document is in edit mode will select the word. This works in most Microsoft applications.
One handy tool which is not switched on by default in Notes, is a right mouse double click closes a window.
This can be set up by:
File->Preferences->User Preferences and then ticking Right Double click closes window.