input: Write a python function to count the number of unique words in a list of documents.
  assert count_unique_words(['The quick brown fox jumps over the lazy dog', 'The quick brown fox jumps over the lazy dog again', 'The quick brown fox jumps over the lazy dog again and again']) == 12
  assert count_unique_words(['Hello world', 'Hello there', 'General Kenobi']) == 4
  assert count_unique_words(['Python is great', 'Python is dynamic', 'Python is fun']) == 5
output: def count_unique_words(documents):
      unique_words = set()
      for document in documents:
          words = document.split()
          unique_words.update(words)
      return len(unique_words)