Add missing documentation examples for BTreeSet.

As part of the ongoing effort to document all methods with examples,
this commit adds the missing examples for the `BTreeSet` collection
type.

This is part of issue #29348.

src/libcollections/btree/set.rs

@@ -38,6 +38,36 @@ use Bound;
 /// [`Ord`]: ../../core/cmp/trait.Ord.html
 /// [`Cell`]: ../../std/cell/struct.Cell.html
 /// [`RefCell`]: ../../std/cell/struct.RefCell.html
+///
+/// # Examples
+///
+/// ```
+/// use std::collections::BTreeSet;
+///
+/// // Type inference lets us omit an explicit type signature (which
+/// // would be `BTreeSet<&str>` in this example).
+/// let mut books = BTreeSet::new();
+///
+/// // Add some books.
+/// books.insert("A Dance With Dragons");
+/// books.insert("To Kill a Mockingbird");
+/// books.insert("The Odyssey");
+/// books.insert("The Great Gatsby");
+///
+/// // Check for a specific one.
+/// if !books.contains("The Winds of Winter") {
+///     println!("We have {} books, but The Winds of Winter ain't one.",
+///              books.len());
+/// }
+///
+/// // Remove a book.
+/// books.remove("The Odyssey");
+///
+/// // Iterate over everything.
+/// for book in &books {
+///     println!("{}", book);
+/// }
+/// ```
 #[derive(Clone, Hash, PartialEq, Eq, Ord, PartialOrd)]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct BTreeSet<T> {