In all the following methods:
  atk_text_get_text
  atk_text_get_text_after_offset
  atk_text_get_text_at_offset
  atk_text_get_text_before_offset
  atk_text_get_selection

the documentation says they return a string (gchar *). Although you can extrapolate that this is a newly created string as it returns a gchar*, but not a const gchar*, I think that it should be good to explicitly mention it on the documentation, as it can be confusing at a first sight.

I will add a proposal patch in the next comment.