Rollup merge of #25291 - johannhof:let-expressions-example, r=steveklabnik

Maybe it's me, but I really needed an example to understand if let and refutable statements.
Playpen: http://is.gd/mjX3Gf

Let me know if the variable names are too, uh, culinary.

src/doc/reference.md

@@ -3072,6 +3072,20 @@ of a condition expression it expects a refutable let statement. If the value of
 expression on the right hand side of the let statement matches the pattern, the corresponding
 block will execute, otherwise flow proceeds to the first `else` block that follows.
 
+```
+let dish = ("Ham", "Eggs");
+
+// this body will be skipped because the pattern is refuted
+if let ("Bacon", b) = dish {
+    println!("Bacon is served with {}", b);
+}
+
+// this body will execute
+if let ("Ham", b) = dish {
+    println!("Ham is served with {}", b);
+}
+```
+
 ### While let loops
 
 A `while let` loop is semantically identical to a `while` loop but in place of a