doc: improve documentation of gtk_text_iter_inside_word()

doc: improve documentation of gtk_text_iter_inside_word()

There is a possible confusion with the sentence:
"@iter is inside a natural-language word"

The iter should be viewed here as the pointed character (i.e. on the
right of the iter), not as a position between two characters.

Instead of improving the documentation, another solution would have been
to change the implementation so it is interpreted as an iter position
inside a word, i.e. between two characters that are part of a
natural-language word. But maybe some applications rely on the current
implementation.

https://bugzilla.gnome.org/show_bug.cgi?id=727908