Add docs to `Frame::new` (#5670)

* [x] I have followed the instructions in the PR template

crates/egui/src/containers/frame.rs

@@ -163,6 +163,9 @@ impl Frame {
         shadow: Shadow::NONE,
     };
 
+    /// No colors, no margins, no border.
+    ///
+    /// Same as [`Frame::NONE`].
     pub const fn new() -> Self {
         Self::NONE
     }