Bootstrap/Load: Add Words of warning about load order.

Since this file is loaded incredibly early, many functions are not available. Those deeply familiar with how load works might have some idea as to what's safe and what isn't, but it's generally confusing and a little opaque so let's warn people there be dragons.

Props helen, DrewAPicture, jorbin.
Fixes #38650.


git-svn-id: https://develop.svn.wordpress.org/trunk@58944 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/default-filters.php

@@ -3,6 +3,12 @@
  * Sets up the default filters and actions for most
  * of the WordPress hooks.
  *
+ * This file is loaded very early in the bootstrap which
+ * means many functions are not yet available and site
+ * information such as if this is multisite is unknown.
+ * Before using functions besides `add_filter` and
+ * `add_action`, verify things will work as expected.
+ *
  * If you need to remove a default hook, this file will
  * give you the priority to use for removing the hook.
  *