Docs: Link to the "Conditional Tags" article in the Theme Developer Handbook from the descriptions for a variety of core conditional tag functions.

These notations largely serve to direct consumers (of both the source and the parsed code reference) to extended information on individual and related conditional tags throughout WordPress. The changeset also standardizes corresponding DocBlock summaries to use third-person singular verbs.

Notations been added for the following functions:

* comments_open()
* email exists()
* has_excerpt()
* has_post_thumbnail()
* has_tag()
* in_category()
* in_the_loop()
* is_404()
* is_active_sidebar()
* is_active_widget()
* is_admin()
* is_admin_bar_showing()
* is_archive()
* is_attachment()
* is_author()
* is_blog_installed()
* is_category()
* is_comments_popup()
* is_date()
* is_day()
* is_dynamic_sidebar()
* is_feed()
* is_front_page()
* is_home()
* is_local_attachment()
* is_main_query
* is_month()
* is_multi_author
* is_new_day()
* is_page()
* is_page_template()
* is_paged()
* is_plugin_active()
* is_plugin_active_for_network()
* is_plugin_inactive()
* is_plugin_page()
* is_post_type_archive()
* is_preview()
* is_rtl()
* is_search()
* is_single()
* is_singular()
* is_sticky()
* is_tag()
* is_tax()
* is_taxonomy_hierarchical()
* is_time()
* is_trackback()
* is_user_logged_in()
* is_year()
* pings_open()
* post_type_exists()
* taxonomy_exists()
* term_exists()
* username exists()
* wp_attachment_is_image()
* wp_script_is()

Props janalwin.
Fixes #43254.


git-svn-id: https://develop.svn.wordpress.org/trunk@42710 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -671,9 +671,13 @@ function wp_get_http_headers( $url, $deprecated = false ) {
 }
 
 /**
- * Whether the publish date of the current post in the loop is different from the
- * publish date of the previous post in the loop.
- *
+ * Determines whether the publish date of the current post in the loop is different
+ * from the publish date of the previous post in the loop.
+ * 
+ * For more information on this and similar theme functions, check out
+ * the {@link https://developer.wordpress.org/themes/basics/conditional-tags/ 
+ * Conditional Tags} article in the Theme Developer Handbook.
+ * 
  * @since 0.71
  *
  * @global string $currentday  The day of the current post in the loop.
@@ -1382,13 +1386,17 @@ function do_robots() {
 }
 
 /**
- * Test whether WordPress is already installed.
+ * Determines whether WordPress is already installed.
  *
  * The cache will be checked first. If you have a cache plugin, which saves
  * the cache values, then this will work. If you use the default WordPress
  * cache, and the database goes away, then you might have problems.
  *
  * Checks for the 'siteurl' option for whether WordPress is installed.
+ * 
+ * For more information on this and similar theme functions, check out
+ * the {@link https://developer.wordpress.org/themes/basics/conditional-tags/ 
+ * Conditional Tags} article in the Theme Developer Handbook.
  *
  * @since 2.1.0
  *