Add meaningful comments for exported identifiers

[gonzojive]

Many types in the library have poor documentation. This is a request for more comments that are meaningful to very casual OAuth users.

Example

// TokenInfo the token information model interface
	TokenInfo interface {
		New() TokenInfo

		GetClientID() string
		SetClientID(string)
		GetUserID() string
		SetUserID(string)
		GetRedirectURI() string
		SetRedirectURI(string)
		GetScope() string
		SetScope(string)

		GetCode() string
		SetCode(string)
		GetCodeCreateAt() time.Time
		SetCodeCreateAt(time.Time)
		GetCodeExpiresIn() time.Duration
		SetCodeExpiresIn(time.Duration)
		GetCodeChallenge() string
		SetCodeChallenge(string)
		GetCodeChallengeMethod() CodeChallengeMethod
		SetCodeChallengeMethod(CodeChallengeMethod)

		GetAccess() string
		SetAccess(string)
		GetAccessCreateAt() time.Time
		SetAccessCreateAt(time.Time)
		GetAccessExpiresIn() time.Duration
		SetAccessExpiresIn(time.Duration)

		GetRefresh() string
		SetRefresh(string)
		GetRefreshCreateAt() time.Time
		SetRefreshCreateAt(time.Time)
		GetRefreshExpiresIn() time.Duration
		SetRefreshExpiresIn(time.Duration)
	}

Comments should be present and should link to a place that explains the relevant concepts (e.g., code challenge, access, refresh, etc.).