doc: add info on sticky vs toggle modifier behavior (#4251)

Author: eugenesvk

winit-core/src/keyboard.rs

@@ -1693,6 +1693,7 @@ bitflags! {
     ///
     /// Note that the modifier key can be physically released with the modifier
     /// still being marked as active, as in the case of sticky modifiers.
+    /// See [`ModifiersKeyState`] for more details on what "sticky" means.
     #[derive(Default, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
     #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
     pub struct ModifiersState: u32 {
@@ -1732,6 +1733,29 @@ impl ModifiersState {
 }
 
 /// The logical state of the particular modifiers key.
+///
+/// NOTE: while the modifier can only be in a binary active/inactive state, it might be helpful to
+/// note the context re. how its state changes by physical key events.
+///
+/// `↓` / `↑` denote physical press/release[^1]:
+///
+/// | Type              | Activated           | Deactivated | Comment |
+/// | ----------------- | :-----------------: | :---------: | ------- |
+/// | __Regular__       | `↓`                 | `↑`         | Active while being held |
+/// | __Sticky__        | `↓`                 | `↓` unless lock is enabled<br>`↓`/`↑`[^2] __non__-sticky key | Temporarily "stuck"; other `Sticky` keys have no effect |
+/// | __Sticky Locked__ | `↓` <br>if `Sticky` | `↓`         | Similar to `Toggle`, but deactivating `↓` turns on `Regular` effect |
+/// | __Toggle__        | `↓`                 | `↓`         | `↑` from the activating `↓` has no effect |
+///
+/// `Sticky` effect avoids the need to press and hold multiple modifiers for a single shortcut and
+/// is usually a platform-wide option that affects modifiers _commonly_ used in shortcuts:
+/// <kbd>Shift</kbd>, <kbd>Control</kbd>, <kbd>Alt</kbd>, <kbd>Meta</kbd>.
+///
+/// `Toggle` type is typically a property of a modifier, for example, <kbd>Caps Lock</kbd>.
+///
+/// These active states are __not__ differentiated here.
+///
+/// [^1]: For virtual/on-screen keyboards physical press/release can be a mouse click or a finger tap or a voice command.
+/// [^2]: platform-dependent
 #[derive(Default, Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub enum ModifiersKeyState {