Twenty Seventeen: Document the the_content filter.

Adds documentation references to the filter's docblock in `wp-includes/post-template.php`.

Follow-up to [38833].

Props pmbaldha, mukesh27, sabernhardt.
Fixes #63637.


git-svn-id: https://develop.svn.wordpress.org/trunk@60430 602fd350-edb4-49c9-b593-d223f7449a82

Author: peterwilsoncc

src/wp-content/themes/twentyseventeen/template-parts/post/content-audio.php

@@ -42,6 +42,7 @@
 	</header><!-- .entry-header -->
 
 	<?php
+		/** This filter is documented in wp-includes/post-template.php */
 		$content = apply_filters( 'the_content', get_the_content() );
 		$audio   = false;
 

src/wp-content/themes/twentyseventeen/template-parts/post/content-video.php

@@ -42,6 +42,7 @@
 	</header><!-- .entry-header -->
 
 	<?php
+		/** This filter is documented in wp-includes/post-template.php */
 		$content = apply_filters( 'the_content', get_the_content() );
 		$video   = false;