unstable book section for `fn_must_use`

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+# `fn_must_use`
+
+The tracking issue for this feature is [#43302].
+
+[#43302]: https://github.com/rust-lang/rust/issues/43302
+
+------------------------
+
+The `fn_must_use` feature allows functions and methods to be annotated with
+`#[must_use]`, indicating that the `unused_must_use` lint should require their
+return values to be used (similarly to how types annotated with `must_use`,
+most notably `Result`, are linted if not used).
+
+## Examples
+
+```rust
+#![feature(fn_must_use)]
+
+#[must_use]
+fn double(x: i32) -> i32 {
+    2 * x
+}
+
+fn main() {
+    double(4); // warning: unused return value of `double` which must be used
+
+    let _ = double(4); // (no warning)
+}
+
+```