[#302] pkg/netmap: Document default values set in NewNetworkInfo

Document field values of instance constructed via `NewNetworkInfo`.
Assert the values in corresponding unit test.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

pkg/netmap/network_info.go

@@ -16,6 +16,10 @@ func NewNetworkInfoFromV2(iV2 *netmap.NetworkInfo) *NetworkInfo {
 }
 
 // NewNetworkInfo creates and initializes blank NetworkInfo.
+//
+// Defaults:
+//  - curEpoch: 0;
+//  - magicNum: 0.
 func NewNetworkInfo() *NetworkInfo {
 	return NewNetworkInfoFromV2(new(netmap.NetworkInfo))
 }