Provide some guidance on what conditionals can/should be used in default-filters.php

[helen]

There are a lot of things you can't use in default-filters.php, such as get_option() or is_customize_preview(). 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. I'd suggest some kind of inline comment at the top explaining the point of the file and what's generally safe (i.e. stuff in load.php as I understand?).

[DrewAPicture]

@helen How specific would you imagine this blurb being? I mean, we can generally outline that the file is loaded after specific files have loaded, e.g. load, class-wp, wp-db, etc. Might also mention that it's effectively loaded before multisite is initialized. I'm not just sure how you envision of the structure of the comment going.

[jorbin]

I think a small note of caution makes sense here. I've added a first draft at the wording, but open to feedback from others.

[jorbin]

In 58944:

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.