docs: add Pushbullet, Pushover, Slack, Telegram (#1268) [skip ci]

* docs: add Pushbullet, Pushover, Telegram

* docs: correction based on feedback from @costaht

* docs: add Slack

* docs: update Telegram docs to reflect changes in #1172

* docs: clarify PGP key format

* docs: edit Pushover documentation

docs/using-overseerr/notifications/discord.md

@@ -1,11 +1,25 @@
 # Discord
 
+{% hint style="info" %}
+The following notification types will mention _all_ users with the **Manage Requests** permission, as these notification types are intended for application administrators rather than end users:
+
+- Media Requested
+- Media Automatically Approved
+- Media Failed
+
+On the other hand, the notification types below will only mention the user who submitted the request:
+
+- Media Approved (does not include automatic approvals)
+- Media Declined
+- Media Available
+
+In order for users to be mentioned in Discord notifications, they must have their [Discord user ID](https://support.discord.com/hc/en-us/articles/206346498-Where-can-I-find-my-User-Server-Message-ID-) configured and **Enable Mentions** checked in their Discord notification user settings.
+{% endhint %}
+
 ## Configuration
 
 {% hint style="info" %}
-In order to configure Discord notifications, you first need to [create a webhook](https://support.discord.com/hc/en-us/articles/228383668-Intro-to-Webhooks).
-
-In order for users to be mentioned in Discord notifications, they must have their [Discord user ID](https://support.discord.com/hc/en-us/articles/206346498-Where-can-I-find-my-User-Server-Message-ID-) configured in their user settings.
+To configure Discord notifications, you first need to [create a webhook](https://support.discord.com/hc/en-us/articles/228383668-Intro-to-Webhooks).
 {% endhint %}
 
 ### Bot Username (optional)