Docs: Add missing descriptions for the $wpdb global in DocBlocks all the places.

See #32246.


git-svn-id: https://develop.svn.wordpress.org/trunk@35170 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/install-helper.php

@@ -154,7 +154,7 @@ function maybe_drop_column($table_name, $column_name, $drop_ddl) {
  *
  * @since 1.0.0
  *
- * @global wpdb $wpdb
+ * @global wpdb $wpdb WordPress database abstraction object.
  *
  * @param string $table_name Table name
  * @param string $col_name   Column name