Explain container iteration in the loop tutorial

 As suggested by @pnkfelix in #12161, this extends the examples by a for-loop
 that iterates over a string as an example for container iteration.

src/doc/tutorial.md

@@ -582,8 +582,7 @@ loop {
 This code prints out a weird sequence of numbers and stops as soon as
 it finds one that can be divided by five.
 
-There is also a for-loop that can be used to iterate over a range of numbers
-(or, more generally, anything implementing the `Iterator` trait):
+There is also a for-loop that can be used to iterate over a range of numbers:
 
 ~~~~
 for n in range(0, 5) {
@@ -593,6 +592,21 @@ for n in range(0, 5) {
 
 The snippet above prints integer numbers under 5 starting at 0.
 
+More generally, a for loop works with anything implementing the `Iterator` trait.
+Data structures can provide one or more methods that return iterators over
+their contents. For example, strings support iteration over their contents in
+various ways:
+
+~~~~
+let s = "Hello";
+for c in s.chars() {
+    println!("{}", c);
+}
+~~~~
+
+The snippet above prints the characters in "Hello" vertically, adding a new
+line after each character.
+
 
 # Data structures