Bug 1873277 - Update acronym dictionary

Author: jonalmeida

fenix/docs/Acronym-dictionary.md

@@ -12,3 +12,6 @@ Explanations of common acronyms that are seen in the Fenix project boards and is
 
 **PWA**: "[Progressive web application](https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps)", Fenix can act as a host for these applications.
 
+**FCM**: "Firebase Cloud Messaging", is the Firebase service used to deliver WebPush messaging to web apps and Firefox Send Tab, to Android devices that have [Google Play Services](https://en.wikipedia.org/wiki/Google_Play_Services).
+
+**PTR**: "[Pull-to-refresh](https://developer.android.com/develop/ui/views/touch-and-input/swipe)", is the feature to refresh the website the user is currently on when pulling down from the top of the page.