Correct exclusive reductions docs: the initial value is not emitted

[adityasingh2400]

The documentation for the exclusive reductions overloads says the returned sequence is "the initial value followed by the reduced elements", but that is not what happens. AsyncExclusiveReductionsSequence.Iterator.next() pulls a base element before emitting anything, so the first emitted element is the result of combining the initial value with the first base element, and the initial value itself is never emitted. For an empty base sequence the result is empty rather than a single element containing the initial value.

This is the behavior the existing tests already encode (for example [1, 2, 3, 4].async.reductions("") { ... } is expected to produce ["1", "12", "123", "1234"], not ["", "1", "12", "123", "1234"]), and changing it would be source-breaking. This corrects the documentation to match the implementation instead, as suggested in the issue.

Changes:

• Rewrite the description and Returns line for all four exclusive overloads (throwing and non-throwing, the value-returning and into: forms) to state that each emitted element is an accumulated result starting from the initial value, that the initial value is not emitted, and that an empty base produces an empty sequence.
• Add test_reductions_empty and test_throwing_reductions_empty covering the empty-base case, which was previously untested and is the most surprising consequence of the corrected wording.

The inclusive reductions doc comments and the DocC Reductions guide already describe the actual behavior, so they are left unchanged.

Testing: swift build and swift test both pass locally (358 tests, including the two new cases). swift format lint --strict is clean on the changed files.

Resolves #386.