Improve atktext documentation
Improve atktext documentation

Previous documentation for these 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

Were not clear if the returned string should be freed by the user or not.

Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=588083

collects/atk,GNOME/at-spi2-core,Distrotech/atk,Distrotech/atk,GNOME/at-spi2-core,collects/atk,GNOME/at-spi2-core,Distrotech/atk