Clarify partial sync flag (#3505)

Signed-off-by: Michael Kaplan <[email protected]>

config/config.md

@@ -176,7 +176,9 @@ must be the same with the number of given `--state-sync-ids`.
 
 #### `--partial-sync-primary-network` (string)
 
-Partial sync enables non-validators to optionally sync only the P-chain on the primary network.
+Partial sync enables nodes that are not primary network validators to optionally sync
+only the P-chain on the primary network. Nodes that use this option can still track 
+Subnets. After the Etna upgrade, nodes that use this option can also validate L1s.
 
 ## Chain Configs