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Update docs/world api.md

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Co-authored-by: Scipio Wright <scipiowright@gmail.com>
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@@ -315,14 +315,16 @@ Access rules usually check for one of two things.
Keep in mind that entrances and locations implicitly check for the accessibility of their parent region, so you do not need to check explicitly for it.
#### An important note on Entrance access rules:
When using `state.can_reach` on an entrance access condition, you must also use `multiworld.register_indirect_condition`.
For efficiency reasons, every time reachable regions are searched, every entrance is only checked once in a somewhat non-deterministic order.
This is fine when checking for items using `state.has`, because items do not change during a region sweep.
However, `state.can_reach` checks for the very same thing we are updating: Regions.
This can lead to non-deterministic behavior and, in the worst case, even generation failures.
Even doing `state.can_reach_location` or `state.can_reach_entrance` is problematic, as these functions call `state.can_reach_region` on the respective parent region.
**Hence, it is considered unsafe to perform `state.can_reach` from within an access condition for an entrance**, unless you are checking for something that sits in the source region of the entrance.
There is a solution to this issue: `multiworld.register_indirect_condition(region, entrance)`. It is an explicit way to tell the generator that when a given region becomes accessible, it is necessary to re-check a specific entrance.
**Therefore, it is considered unsafe to perform `state.can_reach` from within an access condition for an entrance**, unless you are checking for something that sits in the source region of the entrance.
You can use `multiworld.register_indirect_condition(region, entrance)` to explicitly tell the generator that, when a given region becomes accessible, it is necessary to re-check a specific entrance.
You **must** use `multiworld.register_indirect_condition` if you perform this kind of `can_reach` from an entrance access rule, unless you have a **very** good technical understanding of the relevant code and can reason why it will never lead to problems in your case.
### Item Rules