[#197] session: Refactor and document the package

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

session/doc.go

@@ -0,0 +1,48 @@
+/*
+Package session collects functionality of the NeoFS sessions.
+
+Sessions are used in NeoFS as a mechanism for transferring the power of attorney
+of actions to another network member.
+
+Session tokens represent proof of trust. Each session has a limited lifetime and
+scope related to some NeoFS service: Object, Container, etc.
+
+Both parties agree on a secret (private session key), the possession of which
+will be authenticated by a trusted person. The principal confirms his trust by
+signing the public part of the secret (public session key).
+	var tok Container
+	tok.ForVerb(VerbContainerDelete)
+	tok.SetAuthKey(trustedKey)
+	// ...
+
+	err := tok.Sign(principalKey)
+	// ...
+
+	// transfer the token to a trusted party
+
+The trusted member can perform operations on behalf of the trustee.
+
+Instances can be also used to process NeoFS API V2 protocol messages
+(see neo.fs.v2.accounting package in https://github.com/nspcc-dev/neofs-api).
+
+On client side:
+	import "github.com/nspcc-dev/neofs-api-go/v2/session"
+
+	var msg session.Token
+	tok.WriteToV2(&msg)
+
+	// send msg
+
+On server side:
+	// recv msg
+
+	var tok session.Container
+	tok.ReadFromV2(msg)
+
+	// process cnr
+
+Using package types in an application is recommended to potentially work with
+different protocol versions with which these types are compatible.
+
+*/
+package session