explain elseif locality

packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js

@@ -2583,7 +2583,27 @@ export const template_visitors = {
 		];
 
 		if (node.elseif) {
-			// the additional effect layer shouldn't affect local transitions
+			// We treat this...
+			//
+			//   {#if x}
+			//     ...
+			//   {:else}
+			//     {#if y}
+			//       <div transition:foo>...</div>
+			//     {/if}
+			//   {/if}
+			//
+			// ...slightly differently to this...
+			//
+			//   {#if x}
+			//     ...
+			//   {:else if y}
+			//     <div transition:foo>...</div>
+			//   {/if}
+			//
+			// ...even though they're logically equivalent. In the first case, the
+			// transition will only play when `y` changes, but in the second it
+			// should play when `x` or `y` change — both are considered 'local'
 			args.push(b.literal(true));
 		}