update api docs (#4539)

# What this PR does

opened in favour of #4252

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Co-authored-by: Jara Suárez de Puga García <[email protected]>

Author: joeyorlando

docs/sources/oncall-api-reference/_index.md

@@ -37,6 +37,8 @@ Grafana OnCall uses API keys to allow access to the API. You can request a new O
 An API key is specific to a user and a Grafana stack. If you want to switch to a different stack configuration,
 request a different API key.
 
+The endpoint refers to the OnCall Application endpoint and can be found on the OnCall -> Settings page as well.
+
 ## Pagination
 
 List endpoints such as List Integrations or List Alert Groups return multiple objects.