add comment button form element

src/mechanize/form/button.cr

@@ -1,3 +1,5 @@
+# This class represents button related html element.
+# <button>, and <input> whose type is button, reset, image, submit.
 class Mechanize::FormContent::Button < Mechanize::FormContent::Field
   getter form_node : Node | Lexbor::Node
 

src/mechanize/form/image_button.cr

@@ -1,2 +1,3 @@
+# This class represents <input type="image">
 class Mechanize::FormContent::ImageButton < Mechanize::FormContent::Button
 end

src/mechanize/form/reset_button.cr

@@ -1,2 +1,3 @@
+# This class represents <input type="reset">
 class Mechanize::FormContent::ResetButton < Mechanize::FormContent::Button
 end

src/mechanize/form/submit_button.cr

@@ -1,2 +1,3 @@
+# This class represents <input type="submit">
 class Mechanize::FormContent::SubmitButton < Mechanize::FormContent::Button
 end