Fix comments outdated during rust-lang#66648

Author: ssomers

src/liballoc/collections/btree/map.rs

@@ -227,7 +227,7 @@ impl<K: Clone, V: Clone> BTreeClone for BTreeMap<K, V> {
 impl<K: Clone + Ord, V: Clone> BTreeClone for BTreeMap<K, V> {
     fn clone_from(&mut self, other: &Self) {
         // This truncates `self` to `other.len()` by calling `split_off` on
-        // the first key after `other.len()` elements if it exists
+        // the first key after `other.len()` elements if it exists.
         let split_off_key = if self.len() > other.len() {
             let diff = self.len() - other.len();
             if diff <= other.len() {
@@ -247,19 +247,18 @@ impl<K: Clone + Ord, V: Clone> BTreeClone for BTreeMap<K, V> {
         // After truncation, `self` is at most as long as `other` so this loop
         // replaces every key-value pair in `self`. Since `oiter` is in sorted
         // order and the structure of the `BTreeMap` stays the same,
-        // the BTree invariants are maintained at the end of the loop
+        // the BTree invariants are maintained at the end of the loop.
         while !siter.is_empty() {
             if let Some((ok, ov)) = oiter.next() {
-                // SAFETY: This is safe because the `siter.front != siter.back` check
-                // ensures that `siter` is nonempty
+                // SAFETY: This is safe because `siter` is nonempty.
                 let (sk, sv) = unsafe { siter.next_unchecked() };
                 sk.clone_from(ok);
                 sv.clone_from(ov);
             } else {
                 break;
             }
         }
-        // If `other` is longer than `self`, the remaining elements are inserted
+        // If `other` is longer than `self`, the remaining elements are inserted.
         self.extend(oiter.map(|(k, v)| ((*k).clone(), (*v).clone())));
     }
 }