Update browser.md with Mongoose limitations

The current browser.md explains that the browser build is limited, but it
doesn’t clearly list what actually works. This update adds a short note to
make the behavior more understandable.

The note now clarifies that:
• Browser build supports only schema-based validation.
• Built-in validators and custom validators work with validate()/validateSync().
• Setters/getters work as expected.
• Models, queries, defaults, middleware, and all database operations are not
  available in the browser build.

This makes the browser section clearer and reduces confusion for developers
using validation on the frontend.

Author: YashSharma64

docs/browser.md

@@ -34,4 +34,10 @@ const doc = new mongoose.Document({}, new mongoose.Schema({
 }));
 // Prints an error because `name` is required.
 console.log(doc.validateSync());
+> **Note:** The browser version of Mongoose supports only schema-based
+> validation. Built-in validators (like `required`, `enum`, `minlength`) and
+> custom validators work as expected using `validate()` or `validateSync()`.
+> However, browser Mongoose does **not** support models, queries, defaults,
+> middleware (hooks), or any database operations. Only document validation
+> is available in the browser build.
 ```