*: Comment intentional fallthroughs

Falling through a switch case should be commented so it's clear that
behavior is intentional.

src/nvim/buffer.c

@@ -3607,6 +3607,7 @@ int build_stl_str_hl(
 
     case STL_OFFSET_X:
       base = kNumBaseHexadecimal;
+      // fallthrough
     case STL_OFFSET:
     {
       long l = ml_find_line_or_offset(wp->w_buffer, wp->w_cursor.lnum, NULL);
@@ -3617,6 +3618,7 @@ int build_stl_str_hl(
     }
     case STL_BYTEVAL_X:
       base = kNumBaseHexadecimal;
+      // fallthrough
     case STL_BYTEVAL:
       num = byteval;
       if (num == NL)