Replace std::list<ipa::phoneme> with ipa::phonemes

This defines a new ipa::phonemes typedef that is defined to be
std::list<ipa::phoneme>. The rationale for this is twofold:

  1.  it makes it clearer what the intent is;
  2.  it makes it easier to change the representation.
buffer.md: fix the 'operator FILE *' name
docs/api/src: clean up old doxygen \p, \c and \a usage