Improve the documentation for rvalue_static_promotion

Author: tbg

src/doc/unstable-book/src/rvalue-static-promotion.md

@@ -2,4 +2,22 @@
 
 The tracking issue for this feature is: [#38865]
 
+[#38865]: https://github.com/rust-lang/rust/issues/38865
+
 ------------------------
+
+The `rvalue_static_promotion` feature allows directly creating `'static` references to
+constant `rvalue`s, which in particular allowing for more concise code in the common case
+in which a `'static` reference is all that's needed.
+
+
+## Examples
+
+```rust
+#![feature(rvalue_static_promotion)]
+
+fn main() {
+    let DEFAULT_VALUE: &'static u32 = &42;
+    assert_eq!(*DEFAULT_VALUE, 42);
+}
+```

src/test/compile-fail/feature-gate-rvalue_static_promotion.rs

@@ -12,4 +12,4 @@
 fn main() {
     let x: &'static u32 = &42; //~ error: does not live long enough
     let y: &'static Option<u32> = &None; //~ error: does not live long enough
-}
+}

src/test/run-pass/rvalue-static-promotion.rs

@@ -14,4 +14,4 @@
 fn main() {
     let x: &'static u32 = &42;
     let y: &'static Option<u32> = &None;
-}
+}