[docs] Document error scenario for `$Diff<A, B>` utility type
[docs] Document error scenario for `$Diff<A, B>` utility type

Summary:
This commit documents the case in which `$Diff<A, B>` errors when `B` has a key that doesn't exist in `A`.

This is motivated by the following post:
https://fb.facebook.com/groups/flow/permalink/1668785746503367/
https://fb.facebook.com/groups/flow/permalink/1595573580491251/

Reviewed By: calebmer

Differential Revision: D6312634

fbshipit-source-id: 513c13fe47874f40dd8168619b56208fb7b75852

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