fix some docs of prefix lookup api (#1447)

Author: caozhen1937

fluss-client/src/main/java/com/alibaba/fluss/client/lookup/Lookuper.java

@@ -34,8 +34,8 @@ public interface Lookuper {
      * Lookups certain row from the given lookup key.
      *
      * <p>The lookup key must be a primary key if the lookuper is a Primary Key Lookuper (created by
-     * {@code table.newLookuper().create()}), or be the prefix key if the lookuper is a Prefix Key
-     * Lookuper (created by {@code table.newLookuper().withLookupColumns(prefixKeys).create()}).
+     * {@code table.newLookup().createLookuper()}), or be the prefix key if the lookuper is a Prefix
+     * Key Lookuper (created by {@code table.newLookup().lookupBy(prefixKeys).createLookuper()}).
      *
      * @param lookupKey the lookup key.
      * @return the result of lookup.

website/docs/apis/java-client.md

@@ -249,7 +249,17 @@ while (true) {
 ```
 
 ### Lookup
-You can also use the Fluss API to perform lookups on a table. This is useful for querying specific records based on their primary key.
+You can also use the Fluss API to perform lookups on a table. This is useful for querying specific records based on their primary key or prefix key.
 ```java
-LookupResult lookup = table.newLookup().createLookuper().lookup(rowKey).get();
+// Lookup by primary key
+LookupResult lookup = table.newLookup()
+                .createLookuper()
+                .lookup(rowKey)
+                .get();
+// Lookup by prefix key
+LookupResult prefixLookup = table.newLookup()
+        .lookupBy(prefixKeys)
+        .createLookuper()
+        .lookup(rowKey)
+        .get();
 ```