Clarify notice about feature tags being immutable

- Update the recommendation to check for the project running on
  a mobile web browser.

(cherry picked from commit 601eb7799b6d06aa0e42d43458ca927d0978a836)

tutorials/export/feature_tags.rst

@@ -125,15 +125,14 @@ Here is a list of most feature tags in Godot. Keep in mind they are **case-sensi
 
 .. warning::
 
-    With the exception of texture compression and ``movie`` feature tags,
-    default feature tags are **immutable**. This means that they will *not*
-    change depending on run-time conditions. For example,
-    ``OS.has_feature("mobile")`` will return ``false`` when running a project
-    exported to HTML5 on a mobile device.
-
-    To check whether a project exported to HTML5 is running on a mobile device,
-    :ref:`call JavaScript code <doc_web_javascript_bridge>` that reads the browser's
-    user agent.
+    With the exception of texture compression, ``web_<platform>`` and
+    ``movie`` feature tags, default feature tags are **immutable**.
+    This means that they will *not* change depending on run-time conditions.
+    For example, ``OS.has_feature("mobile")`` will return ``false``
+    when running a project exported to Web on a mobile device.
+
+    To check whether a project exported to Web is running on a mobile device,
+    use ``OS.has_feature("web_android") or OS.web_has_feature("web_ios")``.
 
 Custom features
 ---------------