Add @web note to 4.x

Author: AugustMiller

docs/4.x/config/README.md

@@ -315,6 +315,10 @@ Out of the box, Craft provides these aliases—but you can override them or prov
 | `@web` | URL to the folder that contains the `index.php` file that was loaded for the request. | [CRAFT_WEB_URL](#craftweburl)
 | `@webroot` | Path to the folder that contains the `index.php` file that was loaded for the request. | [CRAFT_WEB_ROOT](#craftwebroot)
 
+::: danger
+The `@web` alias is [determined automatically](kb:how-craft-determines-the-site-url), and generally should _not_ be overridden. Additionally, using `@web` in sites’ or asset volumes’ **Base URL** settings can lead to unpredictable behavior.
+:::
+
 Aliases can be set to plain strings, or to the content of an environment variable. Keep in mind that **aliases are resolved recursively**, so you can define one based on another (including those whose values came from the environment):
 
 ```php