DocParser: Use first header in a file as section title

refs #4820

modules/doc/library/Doc/DocParser.php

@@ -72,6 +72,7 @@ class DocParser
                 throw new DocException('Couldn\'t get the lock');
             }
             $line = null;
+            $sectionTitle = null;
             while (! $fileObject->eof()) {
                 // Save last line for setext-style headers
                 $lastLine   = $line;
@@ -79,6 +80,10 @@ class DocParser
                 $header     = $this->extractHeader($line, $lastLine);
                 if ($header !== null) {
                     list($header, $level) = $header;
+                    if ($sectionTitle === null) {
+                        // The first header is the section's title
+                        $sectionTitle = $header;
+                    }
                     $id         = $this->extractHeaderId($header);
                     $nofollow   = false;
                     $this->reduceToc($tocStack, $level);