diff --git a/src/wp-admin/includes/plugin.php b/src/wp-admin/includes/plugin.php
index 0b504b5994..9c4f913548 100644
--- a/src/wp-admin/includes/plugin.php
+++ b/src/wp-admin/includes/plugin.php
@@ -427,7 +427,12 @@ function _get_dropins() {
 }
 
 /**
- * Check whether the plugin is active by checking the active_plugins list.
+ * Check whether a plugin is active.
+ *
+ * Only plugins installed in the plugins/ folder can be active.
+ *
+ * Plugins in the mu-plugins/ folder can't be "activated," so this function will
+ * return false for those plugins.
  *
  * @since 2.5.0
  *
@@ -456,6 +461,11 @@ function is_plugin_inactive( $plugin ) {
 /**
  * Check whether the plugin is active for the entire network.
  *
+ * Only plugins installed in the plugins/ folder can be active.
+ *
+ * Plugins in the mu-plugins/ folder can't be "activated," so this function will
+ * return false for those plugins.
+ *
  * @since 3.0.0
  *
  * @param string $plugin Base plugin path from plugins directory.