Add basic Unstable Book entry for catch_expr.

Author: frewsxcv

src/doc/unstable-book/src/language-features/catch-expr.md

@@ -5,3 +5,26 @@ The tracking issue for this feature is: [#31436]
 [#31436]: https://github.com/rust-lang/rust/issues/31436
 
 ------------------------
+
+The `catch_expr` feature adds support for a `catch` expression. The `catch`
+expression creates a new scope one can use the `?` operator in.
+
+```rust
+#![feature(catch_expr)]
+
+use std::num::ParseIntError;
+
+let result: Result<i32, ParseIntError> = do catch {
+    Ok("1".parse::<i32>()?
+        + "2".parse::<i32>()?
+        + "3".parse::<i32>()?)
+};
+assert_eq!(result, Ok(6));
+
+let result: Result<i32, ParseIntError> = do catch {
+    Ok("1".parse::<i32>()?
+        + "foo".parse::<i32>()?
+        + "3".parse::<i32>()?)
+};
+assert!(result.is_err());
+```