Add universal_impl_trait unstable-book entry

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diff --git a/src/doc/unstable-book/src/language-features/universal-impl-trait.md b/src/doc/unstable-book/src/language-features/universal-impl-trait.md
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+# `universal_impl_trait`
+
+The tracking issue for this feature is: [#34511].
+
+[#34511]: https://github.com/rust-lang/rust/issues/34511
+
+--------------------
+
+The `universal_impl_trait` feature extends the [`conservative_impl_trait`]
+feature allowing the `impl Trait` syntax in arguments (universal
+quantification).
+
+[`conservative_impl_trait`]: ./language-features/conservative-impl-trait.html
+
+## Examples
+
+```rust
+#![feature(universal_impl_trait)]
+use std::ops::Not;
+
+fn any_zero(values: impl IntoIterator<Item = i32>) -> bool {
+    for val in values { if val == 0 { return true; } }
+    false
+}
+
+fn main() {
+    let test1 = -5..;
+    let test2 = vec![1, 8, 42, -87, 60];
+    assert!(any_zero(test1));
+    assert!(bool::not(any_zero(test2)));
+}
+```