update browser-window.md
update browser-window.md

Add a note about the subtle difference between `window.onbeforeunload = handler` and `window.addEventListener('beforeunload', handler)`.
bpasero/electron,thomsonreuters/electron,electron/electron,thomsonreuters/electron,Floato/electron,thomsonreuters/electron,electron/electron,Floato/electron,thomsonreuters/electron,gerhardberger/electron,shiftkey/electron,Floato/electron,shiftkey/electron,electron/electron,the-ress/electron,gerhardberger/electron,electron/electron,shiftkey/electron,seanchas116/electron,gerhardberger/electron,bpasero/electron,bpasero/electron,the-ress/electron,gerhardberger/electron,the-ress/electron,gerhardberger/electron,bpasero/electron,electron/electron,seanchas116/electron,Floato/electron,seanchas116/electron,Floato/electron,shiftkey/electron,seanchas116/electron,seanchas116/electron,bpasero/electron,seanchas116/electron,the-ress/electron,gerhardberger/electron,bpasero/electron,the-ress/electron,shiftkey/electron,electron/electron,shiftkey/electron,thomsonreuters/electron,Floato/electron,thomsonreuters/electron,the-ress/electron,electron/electron,bpasero/electron,the-ress/electron,gerhardberger/electron