Enhance get_comments() phpdoc.

git-svn-id: https://develop.svn.wordpress.org/trunk@11169 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/comment.php

@@ -168,6 +168,8 @@ function &get_comment(&$comment, $output = OBJECT) {
 /**
  * Retrieve a list of comments.
  *
+ * The comment list can be for the blog as a whole or for an individual post.
+ * 
  * The list of comment arguments are 'status', 'orderby', 'comment_date_gmt',
  * 'order', 'number', 'offset', and 'post_id'.
  *