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.