docs: clarify input arrays in device placement

[crusaderky]

Clarify how input arrays should be considered for device placement.
This new line is a repetition of bullet point 2, three lines above:

Preserve device assignment as much as possible (e.g. output arrays from a function are expected to be on the same device as input arrays to the function).

However, I am observing very high levels of ambiguity among the community on the interpretation of this paragraph, where I've had multiple people insist that the device set in the context manager or global default device should trump the device of input arrays. At which point, since all real-life backends always have a global default device, input-to-output device propagation would fly out of the window.

[rgommers]

This change seems reasonable to me, it's more of a clarification than anything else. This numbered list was written with creation functions in mind I believe. The device= keyword descriptions already say something like "if device is None, use the device from the input array" in the relevant cases.

Before merging this, let's ensure the discussions in data-apis/array-api-compat#293 (and perhaps other places) have been settled to ensure it's all consistent.

[crusaderky]

XREF #918
@kgryte ready for review and merge

[kgryte]

LGTM

[kgryte]

Based on linked discussions and workgroup discussion, the proposed rule changes have agreement. As such, I'll go ahead and merge.