style-guide: Add an additional chaining example

Make it clear the rule for stacking the second line on the first applies
recursively, as long as the condition holds.

Author: joshtriplett

src/doc/style-guide/src/expressions.md

@@ -456,12 +456,15 @@ let foo = bar
 
 If the length of the last line of the first element plus its indentation is
 less than or equal to the indentation of the second line, then combine the
-first and second lines if they fit:
+first and second lines if they fit. Apply this rule recursively.
 
 ```rust
 x.baz?
     .qux()
 
+x.y.z
+    .qux()
+
 let foo = x
     .baz?
     .qux();