Docs(lib/coll/btm): Split extract_if's first sentence from the following ones

This also seems like a small mistake: the first main sentence is put in
the same paragraph as the other two following ones while other
equivalents all have it split. Therefore, do the same here.

Signed-off-by: Paul Mabileau <paul.mabileau@harfanglab.fr>

Author: PaulDance

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

@@ -1398,10 +1398,12 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     }
 
     /// Creates an iterator that visits all elements (key-value pairs) in
-    /// ascending key order and uses a closure to determine if an element should
-    /// be removed. If the closure returns `true`, the element is removed from
-    /// the map and yielded. If the closure returns `false`, or panics, the
-    /// element remains in the map and will not be yielded.
+    /// ascending key order and uses a closure to determine if an element
+    /// should be removed.
+    ///
+    /// If the closure returns `true`, the element is removed from the map and
+    /// yielded. If the closure returns `false`, or panics, the element remains
+    /// in the map and will not be yielded.
     ///
     /// The iterator also lets you mutate the value of each element in the
     /// closure, regardless of whether you choose to keep or remove it.