Also pass $key to Document::fromString()

`Document`s sometimes need to know their ID, and passing the `$key` to
`::fromString()` appears to be the least intrusive option to satisfy
this need.

Author: cmb69

classes/Document.php

@@ -41,7 +41,7 @@ interface Document
      *
      * @return ?static
      */
-    public static function fromString(string $contents);
+    public static function fromString(string $contents, string $key);
 
     /**
      * Returns the storage representation of the model object

classes/DocumentStore.php

@@ -115,7 +115,7 @@ public function retrieve(string $key, string $class): ?Document
                 fclose($stream);
             }
         }
-        return $class::fromString($contents ?? "");
+        return $class::fromString($contents ?? "", $key);
     }
 
     /**
@@ -152,7 +152,7 @@ public function update(string $key, string $class): ?Document
                 $contents = "";
             }
         }
-        $document = $class::fromString($contents ?? "");
+        $document = $class::fromString($contents ?? "", $key);
         if ($stream) {
             if ($document === null) {
                 flock($stream, LOCK_UN);

docs/tutorial/persistent_data.rst

@@ -57,7 +57,7 @@ methods:
     {
         private $times = [];
 
-        public static function fromString(string $contents)
+        public static function fromString(string $contents, string $key)
         {
             $that = new static();
             $lines = explode("\n", $contents);
@@ -78,7 +78,7 @@ methods:
         }
     }
 
-`Online::fromString()` parses the file, and stores the relevant
+`Online::fromString()` parses the `$contents`, and stores the relevant
 data in `Online::$times`, while `Online::toString()` reassembles
 the string representation from the private property.
 Next, we implement the three pieces of logic, namely to