docs: address the remaining mailing-list review feedback

[sgerbino]

From the review result:

Corosio: address the documentation comments in this review: https://lists.boost.org/archives/list/boost%40lists.boost.org/message/KHNY67ZNXFWT54EIWDOJPHN4KO6KJLN4

Source: Boost review result (conditional acceptance), Boost mailing list, September 2026. The linked message is "Some thoughts on Corosio" (July 2026).

Items from that message not already tracked in #324:

• Dark mode: black text on dark-blue background is hard to read
• UDP tutorial understates IP fragmentation: fragmented datagrams may be silently discarded (IPv4 practical limit as low as 576 bytes), so oversized datagrams are a correctness hazard, not just a performance note
• Doc examples pervasively pass coroutine arguments by reference/string_view; establish and apply a safe-arguments convention for examples
• "Opening and Closing TCP Connections" draws the branching TCP close sequence as a linear list
• Document executor affinity (class-level thread safety is documented; affinity is not)
• Document the rationale for io_context combining I/O waiting with coroutine execution
• Hash Server tutorial: compute_fnv1a should be a plain function, not a coroutine