doc module: Add `DocIterator::count()'

Method used to count available markdown documentation files.

refs #4820

modules/doc/library/Doc/DocIterator.php

@@ -34,11 +34,12 @@ class FileLockingIterator extends ArrayIterator
     }
 }
 
+use Countable;
 use IteratorAggregate;
 use RecursiveIteratorIterator;
 use RecursiveDirectoryIterator;
 
-class DocIterator implements IteratorAggregate
+class DocIterator implements Countable, IteratorAggregate
 {
     protected $fileInfos;
 
@@ -54,6 +55,11 @@ class DocIterator implements IteratorAggregate
         $this->fileInfos = $fileInfos;
     }
 
+    public function count()
+    {
+        return count($this->fileInfos);
+    }
+
     public function getIterator()
     {
         return new FileLockingIterator($this->fileInfos);