Clarify WebTransport datagram writable support

[KirtiRamchandani]

Description

Adds explicit deprecated/non-standard markers and compatibility notes beside the WebTransport datagram examples that use WebTransportDatagramDuplexStream.writable.

Motivation

The WebTransport.datagrams page can show Baseline support for the datagrams property while the outgoing datagram example uses writable, which has separate compatibility data, is deprecated, is non-standard, and is not supported in Safari. The added notes make that distinction visible where readers encounter the example.

Additional details

The same outgoing datagram example appears on the WebTransportDatagramDuplexStream overview, so this updates both pages consistently.

Related issues and pull requests

Fixes #44273

Tests

• Prettier 3.8.3 check on the two touched pages
• MarkdownLint CLI 0.22.1 with MDN config on the two touched pages
• git diff --check -- files/en-us/web/api/webtransport/datagrams/index.md files/en-us/web/api/webtransportdatagramduplexstream/index.md

[hamishwillee]

@KirtiRamchandani As discussed, it's best not to fix issues that still have the "needs-triage" label - they might not be correct, and even if they are true, the proposed solution might not be optimal.

I've suggested a solution in in #44273 (comment)
I still think your note is also useful in addition, but it probably should mention both the deprecated method and the new alternative, so people can make an informed decision.

[KirtiRamchandani]

Thanks, updated in fe3ec4495e to follow the suggested direction:

• the examples now use transport.datagrams.createWritable() when supported.
• they fall back to transport.datagrams.writable with an inline deprecated/non-standard comment.
• the note now mentions both the newer method and the deprecated property so readers know to check compatibility before choosing a write API.

Local checks:

• corepack npm exec --yes --package prettier@3.8.3 -- prettier --check files/en-us/web/api/webtransport/datagrams/index.md files/en-us/web/api/webtransportdatagramduplexstream/index.md
• corepack npm exec --yes --package markdownlint-cli2@0.22.1 --package markdownlint-rule-search-replace@1.2.0 -- markdownlint-cli2 files/en-us/web/api/webtransport/datagrams/index.md files/en-us/web/api/webtransportdatagramduplexstream/index.md
• git diff --check

[github-actions]

Preview URLs (2 pages)

• /en-US/docs/Web/API/WebTransport/datagrams
• /en-US/docs/Web/API/WebTransportDatagramDuplexStream