Update if-expressions.md (#236)

* Update if-expressions.md

Adding a note about types of `if` branch blocks.

* Fix typo and simplify.

Co-authored-by: Andrew Walbran <qwandor@google.com>

Author: igor-petruk

src/control-flow/if-expressions.md

@@ -25,3 +25,9 @@ fn main() {
     };
 }
 ```
+
+<details>
+
+Because `if` is an expression and must have a particular type, both of its branch blocks must have the same type. Consider showing what happens if you add `;` after `x / 2` in the second example.
+    
+</details>