Add note about earliest_available_slot (#4617)

The formal definition of `earliest_available_slot` has led to some
confusion regarding the implementation, and [requests for
clarification](https://discord.com/channels/595666850260713488/1417614144705794115/1419585033022214144)
have been made.

Author: nalepae

specs/fulu/p2p-interface.md

@@ -319,6 +319,18 @@ As seen by the client at the time of sending the message:
 - `earliest_available_slot`: The slot of earliest available block
   (`SignedBeaconBlock`).
 
+*Note*: According the the definition of `earliest_available_slot`:
+
+- If the node is able to serve all blocks throughout the entire sidecars
+  retention period (as defined by both `MIN_EPOCHS_FOR_BLOB_SIDECARS_REQUESTS`
+  and `MIN_EPOCHS_FOR_DATA_COLUMN_SIDECARS_REQUESTS`), but is NOT able to serve
+  all sidecars during this period, it should advertise the earliest slot from
+  which it can serve all sidecars.
+- If the node is able to serve all sidecars throughout the entire sidecars
+  retention period (as defined by both `MIN_EPOCHS_FOR_BLOB_SIDECARS_REQUESTS`
+  and `MIN_EPOCHS_FOR_DATA_COLUMN_SIDECARS_REQUESTS`), it should advertise the
+  earliest slot from which it can serve all blocks.
+
 ##### BlobSidecarsByRange v1
 
 **Protocol ID:** `/eth2/beacon_chain/req/blob_sidecars_by_range/1/`