*: 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)

src/nvim/edit.c

@@ -4916,13 +4916,16 @@ static unsigned quote_meta(char_u *dest, char_u *src, int len)
       if (ctrl_x_mode == CTRL_X_DICTIONARY
           || ctrl_x_mode == CTRL_X_THESAURUS)
         break;
+      // fallthrough
     case '~':
       if (!p_magic)             /* quote these only if magic is set */
         break;
+      // fallthrough
     case '\\':
       if (ctrl_x_mode == CTRL_X_DICTIONARY
           || ctrl_x_mode == CTRL_X_THESAURUS)
         break;
+      // fallthrough
     case '^':                   /* currently it's not needed. */
     case '$':
       m++;

src/nvim/eval.c

@@ -9596,13 +9596,15 @@ static void f_getcwd(typval_T *argvars, typval_T *rettv, FunPtr fptr)
       if (from) {
         break;
       }
-    case kCdScopeTab:         // FALLTHROUGH
+      // fallthrough
+    case kCdScopeTab:
       assert(tp);
       from = tp->tp_localdir;
       if (from) {
         break;
       }
-    case kCdScopeGlobal:      // FALLTHROUGH
+      // fallthrough
+    case kCdScopeGlobal:
       if (globaldir) {        // `globaldir` is not always set.
         from = globaldir;
       } else if (os_dirname(cwd, MAXPATHL) == FAIL) {  // Get the OS CWD.

src/nvim/ex_getln.c

@@ -1268,6 +1268,7 @@ static int command_line_handle_key(CommandLineState *s)
       }
       return command_line_changed(s);
     }
+    // fallthrough
 
   case K_UP:
   case K_DOWN:

src/nvim/normal.c

@@ -1861,6 +1861,7 @@ void do_pending_operator(cmdarg_T *cap, int old_col, bool gui_yank)
       } else {
         bangredo = true;  // do_bang() will put cmd in redo buffer.
       }
+      // fallthrough
 
     case OP_INDENT:
     case OP_COLON:

src/nvim/regexp_nfa.c

@@ -4194,6 +4194,7 @@ skip_add:
       subs = addstate(l, state->out, subs, pim, off_arg);
       break;
     }
+    // fallthrough
   case NFA_MCLOSE1:
   case NFA_MCLOSE2:
   case NFA_MCLOSE3: