docs: `diff` does not specify type promotion behavior

[asmeurer]

diff() potentially takes multiple arrays as input (x, prepend, and append). But it doesn't specify if they should type promote together, or if the standard is only defined when prepend and append have the same data type as x

https://data-apis.org/array-api/draft/API_specification/generated/array_api.diff.html#diff

[kgryte]

In the current specification guidance, we state that prepend and append should have the same data type as x. Given that prepend and append are kwarg-only and we have yet, TMK, to specify an API in which array kwargs may affect the output dtype, personally, I do not think we should allow type promotion such that the output array may be of a different dtype than x.

However, I think we can clarify, in a note, that if an array library chooses to support prepend and append which are not of the same dtype as x, then behavior is implementation-defined.