docs: clarify shape of values and order of counts in unique APIs

[kgryte]

This PR

• closes unique_all doesn't specify that indices should be a flattened index #588.
• is a follow-up from Clarify flattening behavior and fix required output shape for inverse indices in unique_* APIs #700.
• explicitly states that values in all unique_* APIs must be a one-dimensional array. Previously, the output shape was implied, but not made explicit.
• explicitly states that indices in unique_all must be the indices of a flattened x (ref: unique_all doesn't specify that indices should be a flattened index #588).
• adds a sentence requiring that the order of counts match the order of values.

[rgommers]

LGTM. I verified that this matches the actual behavior of NumPy, and aligns with previous comments on this topic.