docs: document the StreamStatus variants

[Yerimahjr]

Summary

Closes #150. Documents each StreamStatus variant with the exact condition it represents, matching the logic in status().

Changes

• Pending / Streaming / Completed: doc comments now state the precise ledger-time boundary (start_time, end_time) instead of loose phrasing.
• Cancelled: doc comment now states how it interacts with the time-based variants — status() checks cancelled before any time comparison, so once a stream is cancelled it reports Cancelled permanently regardless of ledger time. Also notes cancellation can only happen from Pending or Streaming, since cancel() rejects once now >= end_time.

Scope

Comment-only change in contracts/stream/src/types.rs. No logic, tests, or public API touched.

Test results

• cargo fmt --check — clean
• cargo clippy --all-targets -- -D warnings — clean
• cargo test — 105 passed, 0 failed, 0 ignored

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