Fix warnings while generating RocksJava documentation

Summary:
There are a couple of warnings while building RocksJava, coming from Javadoc generation.
```
Generating target/apidocs/org/rocksdb/RocksDB.html...
src/main/java/org/rocksdb/RocksDB.java:2139: warning: no throws for org.rocksdb.RocksDBException
  public void ingestExternalFile(final List<String> filePathList,
              ^
src/main/java/org/rocksdb/RocksDB.java:2162: warning: no throws for org.rocksdb.RocksDBException
  public void ingestExternalFile(final ColumnFamilyHandle columnFamilyHandle,
              ^
```
Closes https://github.com/facebook/rocksdb/pull/2396

Differential Revision: D5178388

Pulled By: sagar0

fbshipit-source-id: a0ab6696d6de78d089a9a860a559f64cc320019e

java/src/main/java/org/rocksdb/RocksDB.java

@@ -2135,6 +2135,9 @@ public class RocksDB extends RocksObject {
    *
    * @param filePathList The list of files to ingest
    * @param ingestExternalFileOptions the options for the ingestion
+   *
+   * @throws RocksDBException thrown if error happens in underlying
+   *     native library.
    */
   public void ingestExternalFile(final List<String> filePathList,
       final IngestExternalFileOptions ingestExternalFileOptions)
@@ -2158,6 +2161,9 @@ public class RocksDB extends RocksObject {
    * @param columnFamilyHandle The column family for the ingested files
    * @param filePathList The list of files to ingest
    * @param ingestExternalFileOptions the options for the ingestion
+   *
+   * @throws RocksDBException thrown if error happens in underlying
+   *     native library.
    */
   public void ingestExternalFile(final ColumnFamilyHandle columnFamilyHandle,
       final List<String> filePathList,