You can do it in Xcode.
In the line before your class name/method declaration/property declaration, and always in your header (.h) file, you should include a documentation comment (with /// comment
or /** comment **/
). This text will be visible in the Xcode drop down menu whenever you type the name of that object/method/property.
Example:
/// This string contains a certain text.
@property (nonatomic, strong) NSString *string;
or
/** This object subclasses UIView to achieve something. **/
@interface MyView : UIView {
...