AtkText retrieval functions

["Padraig O'Briain" <Padraig Obriain Sun COM>]

I have updated in atk/atktext.c the comments describing the functions 
atk_text_get_text_at_offset(), atk_text_get_text_before_offset() and 
atk_text_get_text_after_offset() to describe what these functions will do.

If anyone finds the comments unclear or disagrees with what I have written 
please speak.

Essentially these functions will return a string which contains the character, 
word, sentence or line at, before or after the specified offset. The return 
string may also contain whitespace before, in the case of END boundary type, or 
after, in the case of START boundary type, the word or sentence.

Padraig