[#164] client: Refactor and document package functionality

Get rid of `Option` pattern. Define `Init`, `Dial` and `Close` methods
for the corresponding stages of use.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

client/doc.go

@@ -0,0 +1,85 @@
+/*
+Package client provides NeoFS API client implementation.
+
+The main component is Client type. It is a virtual connection to the network
+and provides methods for executing operations on the server.
+
+Create client instance:
+	var c client.Client
+
+Initialize client state:
+	var prm client.PrmInit
+	prm.SetDefaultPrivateKey(key)
+	// ...
+
+	c.Init(prm)
+
+Connect to the NeoFS server:
+	var prm client.PrmDial
+	prm.SetServerURI("localhost:8080")
+	prm.SetDefaultPrivateKey(key)
+	// ...
+
+	err := c.Dial(prm)
+	// ...
+
+Execute NeoFS operation on the server:
+	var prm client.PrmContainerPut
+	prm.SetContainer(cnr)
+	// ...
+
+	res, err := c.ContainerPut(context.Background(), prm)
+	err := c.Dial(dialPrm)
+	if err == nil {
+		err = apistatus.ErrFromStatus(res.Status())
+	}
+	// ...
+
+Consume custom service of the server:
+	syntax = "proto3";
+
+	service CustomService {
+		rpc CustomRPC(CustomRPCRequest) returns (CustomRPCResponse);
+	}
+
+	import "github.com/nspcc-dev/neofs-api-go/v2/rpc/client"
+	import "github.com/nspcc-dev/neofs-api-go/v2/rpc/common"
+
+	req := new(CustomRPCRequest)
+	// ...
+	resp := new(CustomRPCResponse)
+
+	err := c.ExecRaw(func(c *client.Client) error {
+		return client.SendUnary(c, common.CallMethodInfo{
+			Service: "CustomService",
+			Name:    "CustomRPC",
+		}, req, resp)
+	})
+	// ...
+
+Close the connection:
+	err := c.Close()
+	// ...
+
+Note that it's not allowed to override Client behaviour directly: the parameters
+for the all operations are write-only and the results of the all operations are
+read-only. To be able to override client behavior (e.g. for tests), abstract it
+with an interface:
+	import "github.com/nspcc-dev/neofs-sdk-go/client"
+
+	type NeoFSClient interface {
+		// Operations according to the application needs
+		CreateContainer(context.Context, container.Container) error
+		// ...
+	}
+
+	type client struct {
+		c *client.Client
+	}
+
+	func (x *client) CreateContainer(context.Context, container.Container) error {
+		// ...
+	}
+
+*/
+package client