update the javadoc

ratis-client/src/main/java/org/apache/ratis/client/api/SnapshotManagementApi.java

@@ -27,16 +27,24 @@
  */
 public interface SnapshotManagementApi {
 
-  /** trigger create snapshot file. */
+  /** The same as create(0, timeoutMs). */
   default RaftClientReply create(long timeoutMs) throws IOException {
     return create(0, timeoutMs);
   }
 
-  /** trigger create snapshot file. If forced, ignore the creation gap. */
+    /** The same as create(force? 1 : 0, timeoutMs). */
   default RaftClientReply create(boolean force, long timeoutMs) throws IOException {
-    return create(1, timeoutMs);
+    return create(force? 1 : 0, timeoutMs);
   }
 
-  /** trigger create snapshot file if the number of newly applied logs since last snapshot exceeds creationGap. */
+    /**
+   * Trigger to create a snapshot.
+   *
+   * @param creationGap When (creationGap > 0) and (astAppliedIndex - lastSnapshotIndex < creationGap),
+   *                    return lastSnapshotIndex; otherwise, take a new snapshot and then return its index.
+   *                    When creationGap == 0, use the server configured value as the creationGap.
+   * @return a reply.  When {@link RaftClientReply#isSuccess()} is true,
+   *         {@link RaftClientReply#getLogIndex()} is the snapshot index fulfilling the operation.
+   */
   RaftClientReply create(long creationGap, long timeoutMs) throws IOException;
 }