Generally the best way is to find the text you want to change and search admin/customize/site_texts for it.
The closest thing to an index would be the translation files in Discourse’s source… there are 2 for each language and these files contain nearly all the text in the app. For English they’re client.en.yml and server.en.yml… the user_notifications.user_watching_first_post text is here in server.en.yml, for example.
So you can search these files for text you want to edit, and then search for the key and edit it in admin/customize/site_texts