"halfway a line" is a very confusing phrase

If you Google for this phrase found in the Vim documentation you'll find
almost exclusively hits from the Vim documentation. I think changing
"halfway a line" to "halfway through a line" makes more sense.

There seems to be an pervasive odd use of the word 'halfway' in the
original docs which I'm updating everywhere.

src/nvim/move.c

@@ -1629,10 +1629,10 @@ void scroll_cursor_bot(int min_scroll, int set_topbot)
   curwin->w_valid |= VALID_TOPLINE;
 }
 
-/*
- * Recompute topline to put the cursor halfway the window
- * If "atend" is TRUE, also put it halfway at the end of the file.
- */
+/// Recompute topline to put the cursor halfway across the window
+///
+/// @param atend if TRUE, also put the cursor halfway to the end of the file.
+///
 void scroll_cursor_halfway(int atend)
 {
   int above = 0;