Rollup merge of #40740 - shepmaster:inclusive-range-unstable-doc, r=steveklabnik

Basic documentation for inclusive range syntax

Done so that we can remove mention of this from the stable documentation ⚠️.

src/doc/unstable-book/src/inclusive-range-syntax.md

@@ -6,5 +6,15 @@ The tracking issue for this feature is: [#28237]
 
 ------------------------
 
+To get a range that goes from 0 to 10 and includes the value 10, you
+can write `0...10`:
 
+```rust
+#![feature(inclusive_range_syntax)]
 
+fn main() {
+    for i in 0...10 {
+        println!("{}", i);
+    }
+}
+```