gg: add doc comments for the FN** types (#25814)

Author: Linklancien

vlib/gg/gg.v

@@ -5,22 +5,31 @@ module gg
 
 pub type FNCb = fn (data voidptr)
 
+// FNEvent defines the type of a function that will be called for every event
 pub type FNEvent = fn (e &Event, data voidptr)
 
+// FNEvent2 same as FNEvent with inverted arguments TODO: deprecate this, in favor of event_fn
 pub type FNEvent2 = fn (data voidptr, e &Event)
 
+// FNFail defines the type of a function that will be called when there is a fail
 pub type FNFail = fn (msg string, data voidptr)
 
+// FNKeyDown defines the type of a function that will be called for every key pressed down
 pub type FNKeyDown = fn (c KeyCode, m Modifier, data voidptr)
 
+// FNKeyUp defines the type of a function that will be called for every release of a pressed key
 pub type FNKeyUp = fn (c KeyCode, m Modifier, data voidptr)
 
+// FNMove defines the type of a function that will be called for every mouse move on the screen
 pub type FNMove = fn (x f32, y f32, data voidptr)
 
+// FNClick defines the type of a function that will be called for every mouse click
 pub type FNClick = fn (x f32, y f32, button MouseButton, data voidptr)
 
+// FNUnClick defines the type of a function that will be called every time a mouse button is released
 pub type FNUnClick = fn (x f32, y f32, button MouseButton, data voidptr)
 
+// FNChar defines the type of a function that will be called once per character
 pub type FNChar = fn (c u32, data voidptr)
 
 pub struct PenConfig {