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)
```
}
      }
      return null;
    }
    @Override
    Iterator<Entry<Cut<C>, Range<C>>> entryIterator() {
      if (restriction.isEmpty()) {
        return Iterators.emptyIterator();
      }
      final Iterator<Range<C>> completeRangeItr;
      if (lowerBoundWindow.upperBound.isLessThan(restriction.lowerBound)) {
        return Iterators.emptyIterator();
      } else if (lowerBoundWindow.lowerBound.isLessThan(restriction.lowerBound)) {
        
        completeRangeItr =
            rangesByUpperBound.tailMap(restriction.lowerBound, false).values().iterator();
      } else {
        
        completeRangeItr =
            rangesByLowerBound
                .tailMap(
                    lowerBoundWindow.lowerBound.endpoint(),
                    lowerBoundWindow.lowerBoundType() == BoundType.CLOSED)
                .values()
                .iterator();
      }
      final Cut<Cut<C>> upperBoundOnLowerBounds =
          Ordering.natural()
              .min(lowerBoundWindow.upperBound, Cut.belowValue(restriction.upperBound));
      return new AbstractIterator<Entry<Cut<C>, Range<C>>>() {