message: update create_in_ram docs
diff: @@ -149,7 +149,8 @@ impl IndexBuilder {
  /// Creates a new index using the [`RamDirectory`].
  ///
  /// The index will be allocated in anonymous memory.
  - /// This should only be used for unit tests.
  + /// This is useful for indexing small set of documents
  + /// for instances like unit test or temporary in memory index.
  pub fn create_in_ram(self) -> Result<Index, TantivyError> {
  let ram_directory = RamDirectory::create();
  self.create(ram_directory)
  