Add comment about gc_num_epochs_to_keep

docs/rpc/hardware-rpc.md

@@ -10,6 +10,9 @@ This page covers the minimum and recommended hardware requirements for engaging
 
 For testing your RPC once the node is fully sync'd, see [this example RPC request.](https://docs.near.org/api/rpc/network#node-status)
 
+The `gc_num_epochs_to_keep` config parameter controls how many epochs are stored on your RPC node.
+The default is 5, with a minimum of 3. Increasing it requires more SSD storage and keeps data for a longer period.
+
 # Mainnet
 
 ## Recommended Hardware Specifications {#recommended-hardware-specifications}