[BEAM-7607] Per user request, making maxFilesPerBundle public (#9160)

* Per user request, making maxFilesPerBundle public

* Adding documentation.

* Apply spotless

sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/BigQueryIO.java

@@ -2019,8 +2019,17 @@ public Write<T> withTestServices(BigQueryServices testServices) {
       return toBuilder().setBigQueryServices(testServices).build();
     }
 
-    @VisibleForTesting
-    Write<T> withMaxFilesPerBundle(int maxFilesPerBundle) {
+    /**
+     * Control how many files will be written concurrently by a single worker when using BigQuery
+     * load jobs before spilling to a shuffle. When data comes into this transform, it is written to
+     * one file per destination per worker. When there are more files than maxFilesPerBundle
+     * (DEFAULT: 20), the data is shuffled (i.e. Grouped By Destination), and written to files
+     * one-by-one-per-worker. This flag sets the maximum number of files that a single worker can
+     * write concurrently before shuffling the data. This flag should be used with caution. Setting
+     * a high number can increase the memory pressure on workers, and setting a low number can make
+     * a pipeline slower (due to the need to shuffle data).
+     */
+    public Write<T> withMaxFilesPerBundle(int maxFilesPerBundle) {
       checkArgument(
           maxFilesPerBundle > 0, "maxFilesPerBundle must be > 0, but was: %s", maxFilesPerBundle);
       return toBuilder().setMaxFilesPerBundle(maxFilesPerBundle).build();