Document each CodeRange value

Author: eregon

src/main/java/org/truffleruby/core/rope/CodeRange.java

@@ -17,9 +17,22 @@
 import com.oracle.truffle.api.CompilerDirectives;
 
 public enum CodeRange {
+    /**
+     * Used for {@link NativeRope}, where the bytes can change from real native code.
+     * Also used when building a new {@link Rope} and the code range is unknown.
+     */
     CR_UNKNOWN(0),
+    /**
+     * Only used for ASCII-compatible encodings, when all characters are US-ASCII (7-bit).
+     */
     CR_7BIT(1),
+    /**
+     * All characters are valid, but at least one non-7-bit character.
+     */
     CR_VALID(2),
+    /**
+     * At least one character is not valid in the encoding of that Rope.
+     */
     CR_BROKEN(3);
 
     private final int value;