explain networkID is usually equal to chainID

Author: fjl

src/eth/client.yaml

@@ -71,7 +71,7 @@
         name: Block number
         value: '0x2377'
 - name: net_version
-  summary: Returns the current network ID.
+  summary: Returns the current network ID. This is usually equivalent to the chainID, but may differ from it for some legacy networks or special testnets.
   params: []
   result:
     name: Network ID