def flatten_documents_to_strings(docs):
    """Flattens the given documents in nested form to a string representation:

    [ #docs
        [ #document1
            ['word1','word2','word3'], #sentence1
            ['word1','word2','word3'], #sentence2
        ],
        [ #document2
            ['word1','word2','word3'], #sentence1
            ['word1','word2','word3'], #sentence2
        ]
    ]

    becomes

    [ #docs
        's1_word1 s1_word2 s1_word3 s2_word1 s2_word2 s2_word3', #document1
        's1_word1 s1_word2 s1_word3 s2_word1 s2_word2 s2_word3', #document2
    ]
    """
    strdocs = []
    for doc in docs:
        strdoc = [' '.join(sent) for sent in doc]
        strdoc = ' '.join(strdoc)
        strdocs.append(strdoc)

    return strdocs