Docs: Correct spelling and capitalization in `wp-includes/load.php`:

* Fix typo in `is_blog_admin()` description.
* Capitalize PHP correctly in `enable_wp_debug_mode_checks` filter description.
* Capitalize XML correctly in `wp_is_xml_request()` return value description.

Props stevegrunwell.
Fixes #48771.
Built from https://develop.svn.wordpress.org/trunk@46759


git-svn-id: https://core.svn.wordpress.org/trunk@46559 1a063a9b-81f0-0310-95a4-ce76da25c4cd