chore: add a note what happens if no compatibility date is sent

docs/services/esi/overview.md

@@ -11,8 +11,13 @@ While some ESI endpoints are public, many require authentication, which is handl
 
 ## Versioning
 
-Every ESI request can include an `X-Compatibility-Date` header using the ISO format - `YYYY-MM-DD`. This header tells ESI "This application was built or updated at this date - give me the API behavior as it was at that date". If applications cannot set custom headers, the `compatibility_date` query parameter will do the same.
-The date cannot be in the future, neither can it below a minimum threshold (the "oldest" versions available). If this minimum bar is raised, this will be clearly communicated via dev-blogs.
+Every ESI request can include an `X-Compatibility-Date` header using the ISO format - `YYYY-MM-DD`.
+This header tells ESI "This application was built or updated at this date - give me the API behavior as it was at that date".
+If applications cannot set custom headers, the `compatibility_date` query parameter will do the same.
+If a request does not set a compatibility date, the oldest available compatibility date is used.
+
+The date cannot be in the future, neither can it below a minimum threshold (the "oldest" versions available).
+If this minimum bar is raised, this will be clearly communicated via dev-blogs.
 
 The API changes date at 11:00 UTC.
 So if you want to use the date of today, use (pseudo-code): `now() - 11 * 60 * 60`, to get the current date of the API.