Added comments for ICaptcha interface.

pkg/captcha/captcha.go

@@ -16,15 +16,33 @@ var errorNotFound = errors.New("captcha not found")
 
 type Answer string
 
+// ICaptcha interface that should be implemented by captcha.
 type ICaptcha interface {
+	// generateImage is used to create a captcha image.
 	generateImage() *image.Image
+	// GetImage() returns an image of captcha. Calls
+	// generateImage() method if image doesn't exist.
 	GetImage() *image.Image
+	// GetAnswer returns a pregenerated answer.
 	GetAnswer() Answer
+	// Solve compares a stored answer with a passed one.
+	// Sets field Solved to true if they are equal.
 	Solve(answer Answer) bool
+	// IsSolved returns field IsSolved.
 	IsSolved() bool
+	// Expiry returns a date after what captcha will expire.
 	Expiry() time.Time
 }
 
+// Captcha is a base implementation of a CAPTCHA.
+//
+// All derivatives that embed this struct only need to
+// implement GetImage() and generateImage() methods.
+// And GetImage() could simply be copied.
+//
+// You need to do that because in Go there is no inheritance,
+// and if you won't do that, calling a base GetImage
+// method will call an empty base generateImage() method.
 type Captcha struct {
 	Image    image.Image
 	Answer   Answer