Document that hardware back button event for overlays does not fire if overlay has backdropDismiss set to false

**URL**
https://ionicframework.com/docs/developing/hardware-back-button

**What is missing or inaccurate about the content on this page?**
Just spent a few hours debugging why my registration for hardware back button, which shows an alert dialog, keeps firing even when the alert dialog was displayed, essentially showing an alert on top of an alert, although I registered it with priority 1 (which is well bellow the 100 assigned to overlays).

Turns out the overlay is registering for back button handling only if backdropDismiss is set to true, and this is NOT mentioned in the docs at all.
See [here](https://github.com/ionic-team/ionic/blob/1decc13cb8d355a6fa32d303271a34550082cada/core/src/utils/overlays.ts#L66) for the actual logic.