tests textiter: test word boundaries

- As the tests show, some of the functions have a strange and
inconsistent behavior for corner cases.

- Rename test_full_buffer() -> test_search_full_buffer() because
textiter.c is used for other GtkTextIter unit tests.
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.