Plugins installed in the plugins/ folder can be activated by users. This puts the plugin in an "active" state. Plugins in the mu-folder/ are "must-use" plugins that are always loaded. They can't be activated, and thereby can't be "active." Because of this, using the `is_plugin_active()` or `is_plugin_active_for_network()` returns false when checking for these plugins. This clarifies that behavior in the functions' docs.
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