Clarify max_total_wal_size's scope (#4194)

Summary:
max_total_wal_size takes effect only when there are more than one column families. The patch clarify that in the inline docs

Closes https://github.com/facebook/rocksdb/issues/4180
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4194

Differential Revision: D9028767

Pulled By: maysamyabandeh

fbshipit-source-id: 8d730ca7f15e76e7ee9ff88b2b48030b2d1b7078

include/rocksdb/options.h

@@ -430,6 +430,8 @@ struct DBOptions {
   // (i.e. the ones that are causing all the space amplification). If set to 0
   // (default), we will dynamically choose the WAL size limit to be
   // [sum of all write_buffer_size * max_write_buffer_number] * 4
+  // This option takes effect only when there are more than one column family as
+  // otherwise the wal size is dictated by the write_buffer_size.
   // Default: 0
   uint64_t max_total_wal_size = 0;
 

java/src/main/java/org/rocksdb/DBOptionsInterface.java

@@ -262,6 +262,8 @@ public interface DBOptionsInterface<T extends DBOptionsInterface> {
    * </p>
    * <p>If set to 0 (default), we will dynamically choose the WAL size limit to
    * be [sum of all write_buffer_size * max_write_buffer_number] * 2</p>
+   * <p>This option takes effect only when there are more than one column family as
+   * otherwise the wal size is dictated by the write_buffer_size.</p>
    * <p>Default: 0</p>
    *
    * @param maxTotalWalSize max total wal size.