Lightly tweak docs for BTree{Map,Set}::extract_if

- Move explanations into comments to match style
- Explain the second examples
- Make variable names match the data structure

Related #70530

Author: rs-sac

library/alloc/src/collections/btree/map.rs

@@ -1416,18 +1416,18 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     ///
     /// # Examples
     ///
-    /// Splitting a map into even and odd keys, reusing the original map:
-    ///
     /// ```
     /// #![feature(btree_extract_if)]
     /// use std::collections::BTreeMap;
     ///
+    /// // Splitting a map into even and odd keys, reusing the original map:
     /// let mut map: BTreeMap<i32, i32> = (0..8).map(|x| (x, x)).collect();
     /// let evens: BTreeMap<_, _> = map.extract_if(.., |k, _v| k % 2 == 0).collect();
     /// let odds = map;
     /// assert_eq!(evens.keys().copied().collect::<Vec<_>>(), [0, 2, 4, 6]);
     /// assert_eq!(odds.keys().copied().collect::<Vec<_>>(), [1, 3, 5, 7]);
     ///
+    /// // Splitting a map into low and high halves, reusing the original map:
     /// let mut map: BTreeMap<i32, i32> = (0..8).map(|x| (x, x)).collect();
     /// let low: BTreeMap<_, _> = map.extract_if(0..4, |_k, _v| true).collect();
     /// let high = map;

library/alloc/src/collections/btree/set.rs

@@ -1201,21 +1201,21 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// [`retain`]: BTreeSet::retain
     /// # Examples
     ///
-    /// Splitting a set into even and odd values, reusing the original set:
-    ///
     /// ```
     /// #![feature(btree_extract_if)]
     /// use std::collections::BTreeSet;
     ///
+    /// // Splitting a set into even and odd values, reusing the original set:
     /// let mut set: BTreeSet<i32> = (0..8).collect();
     /// let evens: BTreeSet<_> = set.extract_if(.., |v| v % 2 == 0).collect();
     /// let odds = set;
     /// assert_eq!(evens.into_iter().collect::<Vec<_>>(), vec![0, 2, 4, 6]);
     /// assert_eq!(odds.into_iter().collect::<Vec<_>>(), vec![1, 3, 5, 7]);
     ///
-    /// let mut map: BTreeSet<i32> = (0..8).collect();
-    /// let low: BTreeSet<_> = map.extract_if(0..4, |_v| true).collect();
-    /// let high = map;
+    /// // Splitting a set into low and high halves, reusing the original set:
+    /// let mut set: BTreeSet<i32> = (0..8).collect();
+    /// let low: BTreeSet<_> = set.extract_if(0..4, |_v| true).collect();
+    /// let high = set;
     /// assert_eq!(low.into_iter().collect::<Vec<_>>(), [0, 1, 2, 3]);
     /// assert_eq!(high.into_iter().collect::<Vec<_>>(), [4, 5, 6, 7]);
     /// ```