[#111] client: Add comments to `Client` and `New`

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

client/client.go

@@ -4,12 +4,26 @@ import (
 	"github.com/nspcc-dev/neofs-api-go/v2/rpc/client"
 )
 
+// Client is a wrapper over raw NeoFS API client.
+//
+// It is not allowed to override client's behaviour:
+// the parameters for the all operations are write-only
+// and the results of the all operations are read-only.
+//
+// Working client must be created via constructor New.
+// Using the Client that has been created with new(Client)
+// expression (or just declaring a Client variable) is unsafe
+// and can lead to panic.
 type Client struct {
 	raw *client.Client
 
 	opts *clientOptions
 }
 
+// New creates, initializes and returns the Client instance.
+//
+// If multiple options of the same config value are supplied,
+// the option with the highest index in the arguments will be used.
 func New(opts ...Option) (*Client, error) {
 	clientOptions := defaultClientOptions()