docs(db-mongodb): note on indexing localized fields & per-locale growth (#13469)

Closes #13464

Adds a note to the Indexes docs for localized fields:
- Indexing a `localized: true` field creates one index per locale path
(e.g. `slug.en`, `slug.da-dk`), which can grow index count on MongoDB.
- Recommends defining explicit indexes via collection-level `indexes`
for only the locale paths you actually query.
- Includes a concrete example (index `slug.en` only).

Docs-only change.

---------

Co-authored-by: German Jablonski <43938777+GermanJablo@users.noreply.github.com>

Author: chenxi-debugger

docs/database/indexes.mdx

@@ -63,3 +63,22 @@ export const MyCollection: CollectionConfig = {
   ],
 }
 ```
+## Localized fields and MongoDB indexes
+
+When you set `index: true` or `unique: true` on a localized field, MongoDB creates one index **per locale path** (e.g., `slug.en`, `slug.da-dk`, etc.). With many locales and indexed fields, this can quickly approach MongoDB's per-collection index limit.
+
+If you know you'll query specifically by a locale, index only those locale paths using the collection-level `indexes` option instead of setting `index: true` on the localized field. This approach gives you more control and helps avoid unnecessary indexes.
+
+```ts
+import type { CollectionConfig } from 'payload'
+
+export const Pages: CollectionConfig = {
+  fields: [{ name: 'slug', type: 'text', localized: true }],
+  indexes: [
+    // Index English slug only (rather than all locales)
+    { fields: ['slug.en'] },
+    // You could also make it unique:
+    // { fields: ['slug.en'], unique: true },
+  ],
+}
+```