Added doc on keyword break

Author: dorfsmay

src/libstd/keyword_docs.rs

@@ -33,9 +33,71 @@ mod as_keyword { }
 //
 /// Exit early from a loop.
 ///
-/// The documentation for this keyword is [not yet complete]. Pull requests welcome!
+/// When `break` is encountered, execution of the associated loop body is
+/// immediately terminated.
+///
+/// ```rust
+/// let mut last = 0;
+///
+/// for x in 1..100 {
+///     if x > 12 {
+///         break;
+///     }
+///     last = x;
+/// }
+///
+/// assert_eq!(last, 12);
+/// println!("{}", last);
+/// ```
+///
+/// A break expression is normally associated with the innermost loop enclosing the
+/// `break` but a label can be used to specify which enclosing loop is affected.
+///
+///```rust
+/// 'outer: for i in 1..=5 {
+///     println!("outer iteration (i): {}", i);
+///
+///     'inner: for j in 1..=200 {
+///         println!("    inner iteration (j): {}", j);
+///         if j >= 3 {
+///             // breaks from inner loop, let's outer loop continue.
+///             break;
+///         }
+///         if i >= 2 {
+///             // breaks from outer loop, and directly to "Bye".
+///             break 'outer;
+///         }
+///     }
+/// }
+/// println!("Bye.");
+///```
+///
+/// When associated with `loop`, but not with any other kind of loop expression,
+/// `break` can return a value.  When no value is specified, `break;` returns `()`.
+/// Every `break` within a loop must return the same type.
+///
+/// ```rust
+/// let (mut a, mut b) = (1, 1);
+/// let result = loop {
+///     if b > 10 {
+///         break b;
+///     }
+///     let c = a + b;
+///     a = b;
+///     b = c;
+/// };
+/// // first number in Fibonacci sequence over 10:
+/// assert_eq!(result, 13);
+/// println!("{}", result);
+/// ```
+///
+/// For more details consult the [Reference on "break expression"] and the [Reference on "break and
+/// loop values"].
+///
+/// [Reference on "break expression"]: ../reference/expressions/loop-expr.html#break-expressions
+/// [Reference on "break and loop values"]:
+/// ../reference/expressions/loop-expr.html#break-and-loop-values
 ///
-/// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
 mod break_keyword { }
 
 #[doc(keyword = "const")]