"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/ops.c

@@ -517,7 +517,7 @@ static void block_insert(oparg_T *oap, char_u *s, int b_insert, struct block_def
     if (has_mbyte && spaces > 0) {
       int off;
 
-      // Avoid starting halfway a multi-byte character.
+      // Avoid starting halfway through a multi-byte character.
       if (b_insert) {
         off = (*mb_head_off)(oldp, oldp + offset + spaces);
       } else {