Fix misleading docblocks (#33111)

Author: nikazooz

src/Illuminate/Collections/EnumeratesValues.php

@@ -518,7 +518,7 @@ public function where($key, $operator = null, $value = null)
     }
 
     /**
-     * Filter items where the given key is not null.
+     * Filter items where the value for the given key is null.
      *
      * @param  string|null  $key
      * @return static
@@ -529,7 +529,7 @@ public function whereNull($key = null)
     }
 
     /**
-     * Filter items where the given key is null.
+     * Filter items where the value for the given key is not null.
      *
      * @param  string|null  $key
      * @return static