Fix comment in treesit_record_change (bug#61369)

Fix comment in treesit_record_change (bug#61369)

Turns out the previous commit message and comment is not entirely
correct: the old behavior is in fact wrong, not just "correct but has
problems".

Here is why the old code is wrong:

|visible range|     -> markup for visible range

updated range       -> markup for updated range
-------------

First we have some text

|aaaaaa|

Now we insert something at the beginning, because we clip
new_end_offset to visible_end, out of eight b's inserted, only the
first six are known to tree-sitter.

|bbbbbbbbaaaa|aa  start: 0, old_end: 0, new_end: 6
 ------

In treesit_sync_visible_region, we sync up visible region, but the two
missing b's are not in the updated range.

|bbbbbbbbaaaaaa|  start: 12, old_end: 12, new_end: 14
             --

The old behavior not only is wrong, but also doesn't make much sense.

* src/treesit.c (treesit_record_change): Update comment.