Networks and sites: Replace "blog" usage with "site" in docs.

Multisite functions use the term "blog" to refer to what we now call a "site," e.g. `get_current_blog_id()`. These functions are here to stay because of our commitment to backwards compatibility. What we can do is set the documentation straight.

See #35417.


git-svn-id: https://develop.svn.wordpress.org/trunk@36417 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -1334,7 +1334,7 @@ function do_robots() {
 }
 
 /**
- * Test whether blog is already installed.
+ * Test 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
@@ -1346,7 +1346,7 @@ function do_robots() {
  *
  * @global wpdb $wpdb WordPress database abstraction object.
  *
- * @return bool Whether the blog is already installed.
+ * @return bool Whether the site is already installed.
  */
 function is_blog_installed() {
 	global $wpdb;