Unable to modify subRangeSet returned by TreeRangeSet.subRangeSet(Range).
IMO, it should be availiable to do so as specified in the document.

```java
import com.google.common.collect.Range;
import com.google.common.collect.TreeRangeSet;
import org.junit.jupiter.api.Test;

@SuppressWarnings("UnstableApiUsage")
public class SubRangeSetTest {

    @Test
    public void testAdd() {
        TreeRangeSet<Integer> set = TreeRangeSet.create();
        Range<Integer> range = Range.closedOpen(0, 5);
        set.subRangeSet(range).add(range);
    }

    @Test
    public void testReplaceAdd() {
        TreeRangeSet<Integer> set = TreeRangeSet.create();
        Range<Integer> range = Range.closedOpen(0, 5);
        set.add(range);
        set.subRangeSet(range).add(range);
    }
}
```

```
java.lang.UnsupportedOperationException
	at java.base/java.util.AbstractMap.put(AbstractMap.java:209)
	at com.google.common.collect.TreeRangeSet.replaceRangeWithSameLowerBound(TreeRangeSet.java:266)
	at com.google.common.collect.TreeRangeSet.add(TreeRangeSet.java:216)
	at com.google.common.collect.TreeRangeSet$SubRangeSet.add(TreeRangeSet.java:894)
	at SubRangeSetTest.testAdd(SubRangeSetTest.java:12)
```
```
java.lang.UnsupportedOperationException
	at com.google.common.collect.UnmodifiableIterator.remove(UnmodifiableIterator.java:46)
	at com.google.common.collect.Iterators.clear(Iterators.java:985)
	at com.google.common.collect.Maps$IteratorBasedAbstractMap.clear(Maps.java:3562)
	at com.google.common.collect.TreeRangeSet.add(TreeRangeSet.java:214)
	at com.google.common.collect.TreeRangeSet$SubRangeSet.add(TreeRangeSet.java:894)
	at SubRangeSetTest.testReplaceAdd(SubRangeSetTest.java:20)
```
private final Range<C> restriction;
    SubRangeSet(Range<C> restriction) {
      super(
          new SubRangeSetRangesByLowerBound<C>(
              Range.<Cut<C>>all(), restriction, TreeRangeSet.this.rangesByLowerBound));
      this.restriction = restriction;
    }
    @Override
    public boolean encloses(Range<C> range) {
      if (!restriction.isEmpty() && restriction.encloses(range)) {
        Range<C> enclosing = TreeRangeSet.this.rangeEnclosing(range);
        return enclosing != null && !enclosing.intersection(restriction).isEmpty();
      }
      return false;
    }
    @Override
    @NullableDecl
    public Range<C> rangeContaining(C value) {
      if (!restriction.contains(value)) {
        return null;
      }
      Range<C> result = TreeRangeSet.this.rangeContaining(value);
      return (result == null) ? null : result.intersection(restriction);
    }
    @Override
    public void add(Range<C> rangeToAdd) {
      checkArgument(
          restriction.encloses(rangeToAdd),
          "Cannot add range %s to subRangeSet(%s)",
          rangeToAdd,