document what the span of UseTreeKind::Nested is

Author: pietroalbini

compiler/rustc_ast/src/ast.rs

@@ -2733,6 +2733,13 @@ pub enum UseTreeKind {
     /// `use prefix` or `use prefix as rename`
     Simple(Option<Ident>),
     /// `use prefix::{...}`
+    ///
+    /// The span represents the braces of the nested group and all elements within:
+    ///
+    /// ```text
+    /// use foo::{bar, baz};
+    ///          ^^^^^^^^^^
+    /// ```
     Nested { items: ThinVec<(UseTree, NodeId)>, span: Span },
     /// `use prefix::*`
     Glob,