Add iinvoice type documentation

Author: ilyakaznacheev

types.go

@@ -1633,11 +1633,20 @@ type InputContactMessageContent struct {
 
 // Invoice contains basic information about an invoice.
 type Invoice struct {
-	Title          string `json:"title"`
-	Description    string `json:"description"`
+	// Title product name
+	Title string `json:"title"`
+	// Description product description
+	Description string `json:"description"`
+	// StartParameter unique bot deep-linking parameter that can be used to generate this invoice
 	StartParameter string `json:"start_parameter"`
-	Currency       string `json:"currency"`
-	TotalAmount    int    `json:"total_amount"`
+	// Currency three-letter ISO 4217 currency code
+	// (see https://core.telegram.org/bots/payments#supported-currencies)
+	Currency string `json:"currency"`
+	// TotalAmount otal price in the smallest units of the currency (integer, not float/double).
+	// For example, for a price of US$ 1.45 pass amount = 145.
+	// See the exp parameter in currencies.json, it shows the number of digits
+	// past the decimal point for each currency (2 for the majority of currencies).
+	TotalAmount int `json:"total_amount"`
 }
 
 // LabeledPrice represents a portion of the price for goods or services.