"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/edit.c

@@ -766,8 +766,8 @@ edit (
 #ifdef UNIX
 do_intr:
 #endif
-      /* when 'insertmode' set, and not halfway a mapping, don't leave
-       * Insert mode */
+      // when 'insertmode' set, and not halfway through a mapping, don't leave
+      // Insert mode
       if (goto_im()) {
         if (got_int) {
           (void)vgetc();                        /* flush all buffers */