docs: add docs for text selection

Still really minimal, but text selection is not obvious, so this is a
lot better than nothing.

Author: luca3s

common/src/lib.rs

@@ -55,6 +55,10 @@ pub use geometry::{Affine, Point, Rect, Size, Vec2};
 pub enum Role {
     #[default]
     Unknown,
+
+    /// Should be used for selectable Text. For each line of text one `TextRun` should be used.
+    /// Each one of them should be added as a child to a `Label`, `TextInput`, `MultiLineTextInput`
+    /// or similar [`Node`]. The selection can be specified with [`Node::set_text_selection`].
     TextRun,
     Cell,
     Label,
@@ -1989,6 +1993,9 @@ property_methods! {
     /// [`transform`]: Node::transform
     (Bounds, bounds, get_rect_property, Option<Rect>, set_bounds, set_rect_property, Rect, clear_bounds),
 
+    /// Text selection should be added to the parent [`Node`] of the `TextRun` nodes.
+    /// In [`TextSelection`] the line of selected text is specified by setting the [`NodeId`]
+    /// to the `TextRun` that holds the selected line.
     (TextSelection, text_selection, get_text_selection_property, Option<&TextSelection>, set_text_selection, set_text_selection_property, impl Into<Box<TextSelection>>, clear_text_selection)
 }