underestimatedCount complexity documentation is broken

[dabrahams]

Description

  /// - Complexity: O(1), except if the sequence also conforms to `Collection`.
  ///   In this case, see the documentation of `Collection.underestimatedCount`.

This was done in ea49459
The correct complexity bound is O(N) where N is the length of the sequence.

The current wording is meaningless, other than to say the complexity bound is O(N) (as given by Collection's requirements). When you are handling a Sequence, (modulo dynamic casts that you couldn't even perform when that change was made and that nobody does in practice), you don't know that it isn't a Collection. Furthermore, nearly any Sequence can be made to conform to Collection post-hoc. I don't know what made anyone think they could tighten a complexity bound on an existing protocol, but (obviously) when you do that you break anyone that had a previously-valid conformance. More generally, it is broken and wrong to loosen constraints in a refinement. In this case that would make every Collection not-a Sequence.

Reproduction

Read the docs.

Expected behavior

The docs make sense and are meaningful.

Environment

Any environment you like

Additional information

No response