Adds missing javadocs for RedisOperations.hasKey().

Also, added missing javadocs about what the method returns and about nullability.

Closes #3049
Original pull request: #3055

Author: marcingrzejszczak

src/main/java/org/springframework/data/redis/core/RedisOperations.java

@@ -47,6 +47,7 @@
  * @author Todd Merrill
  * @author Chen Li
  * @author Vedran Pavic
+ * @author Marcin Grzejszczak
  */
 public interface RedisOperations<K, V> {
 
@@ -151,7 +152,7 @@ <T> T execute(RedisScript<T> script, RedisSerializer<?> argsSerializer, RedisSer
 	 * to free resources after use.
 	 *
 	 * @param callback must not be {@literal null}.
-	 * @return
+	 * @return the {@link Object result} of the operation performed in the callback or {@literal null}.
 	 * @since 1.8
 	 */
 	@Nullable
@@ -167,7 +168,7 @@ <T> T execute(RedisScript<T> script, RedisSerializer<?> argsSerializer, RedisSer
 	 * @param sourceKey must not be {@literal null}.
 	 * @param targetKey must not be {@literal null}.
 	 * @param replace whether the key was copied. {@literal null} when used in pipeline / transaction.
-	 * @return
+	 * @return {@code true} when copied successfully or {@literal null} when used in pipeline / transaction.
 	 * @see <a href="https://redis.io/commands/copy">Redis Documentation: COPY</a>
 	 * @since 2.6
 	 */
@@ -178,7 +179,7 @@ <T> T execute(RedisScript<T> script, RedisSerializer<?> argsSerializer, RedisSer
 	 * Determine if given {@code key} exists.
 	 *
 	 * @param key must not be {@literal null}.
-	 * @return
+	 * @return {@literal true} if key exists. {@literal null} when used in pipeline / transaction.
 	 * @see <a href="https://redis.io/commands/exists">Redis Documentation: EXISTS</a>
 	 */
 	@Nullable
@@ -361,7 +362,7 @@ default Boolean expireAt(K key, Instant expireAt) {
 	 * Remove the expiration from given {@code key}.
 	 *
 	 * @param key must not be {@literal null}.
-	 * @return {@literal null} when used in pipeline / transaction.
+	 * @return {@code true} when persisted successfully or {@literal null} when used in pipeline / transaction.
 	 * @see <a href="https://redis.io/commands/persist">Redis Documentation: PERSIST</a>
 	 */
 	@Nullable