Improve documentation on string normalization

[mcollardanuy]

In utils.py, document actions in normalizeString function (lines 22-33, see here):

  def normalizeString(s, uni2ascii=False, lowercase=False, strip=False, only_latin_letters=False, prefix_suffix=["|", "|"]):
    if uni2ascii:
        s = unicodedata.normalize('NFKD', str(s))
    if lowercase:
        s = s.lower()
    if strip:
        s = s.strip()
    if only_latin_letters:
        s = re.sub(r"([.!?])", r" \1", s)
        s = re.sub(r"[^a-zA-Z.!?]+", r" ", s)

    return prefix_suffix[0] + s + prefix_suffix[1]

[mcollardanuy]

Done here.