Updated documentation in comments at the top of files.

I tried to standardize and expand the documentation and links to
codex.wordpress.org in the comments at the top of most files.

inc/custom-header.php

@@ -1,7 +1,6 @@
 <?php
 /**
- * Sample implementation of the Custom Header feature
- * http://codex.wordpress.org/Custom_Headers
+ * Sample implementation of the Custom Header feature.
  *
  * You can add an optional custom header image to header.php like so ...
  *
@@ -11,6 +10,8 @@
 	</a>
 	<?php endif; // End header image check. ?>
  *
+ * @link http://codex.wordpress.org/Custom_Headers
+ *
  * @package _s
  */
 

inc/customizer.php

@@ -1,6 +1,6 @@
 <?php
 /**
- * _s Theme Customizer
+ * _s Theme Customizer.
  *
  * @package _s
  */

inc/extras.php

@@ -1,8 +1,8 @@
 <?php
 /**
- * Custom functions that act independently of the theme templates
+ * Custom functions that act independently of the theme templates.
  *
- * Eventually, some of the functionality here could be replaced by core features
+ * Eventually, some of the functionality here could be replaced by core features.
  *
  * @package _s
  */

inc/jetpack.php

@@ -1,7 +1,8 @@
 <?php
 /**
- * Jetpack Compatibility File
- * See: https://jetpack.me/
+ * Jetpack Compatibility File.
+ *
+ * @link https://jetpack.me/
  *
  * @package _s
  */