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)
```
}
        } catch (ClassCastException e) {
          return null;
        }
      }
      return null;
    }
    @Override
    Iterator<Entry<Cut<C>, Range<C>>> entryIterator() {
      /*
       * We want to start the iteration at the first range where the upper bound is in
       * upperBoundWindow.
       */
      final Iterator<Range<C>> backingItr;
      if (!upperBoundWindow.hasLowerBound()) {
        backingItr = rangesByLowerBound.values().iterator();
      } else {
        Entry<Cut<C>, Range<C>> lowerEntry =
            rangesByLowerBound.lowerEntry(upperBoundWindow.lowerEndpoint());
        if (lowerEntry == null) {
          backingItr = rangesByLowerBound.values().iterator();
        } else if (upperBoundWindow.lowerBound.isLessThan(lowerEntry.getValue().upperBound)) {
          backingItr = rangesByLowerBound.tailMap(lowerEntry.getKey(), true).values().iterator();
        } else {
          backingItr =
              rangesByLowerBound
                  .tailMap(upperBoundWindow.lowerEndpoint(), true)
                  .values()
                  .iterator();
        }