docs(streaming): document the RetroArch core override

[LoneAngelFayt]

Documents the per-platform RetroArch core override on the Emulator Streaming page and in the configuration reference.

Companion to rommapp/romm#4859 (the core / experimental_cores setting) and rommapp/romm#4862 (save states kept per core). Both need a broker with romm-streaming/romm-broker#72, which is merged. Merge this when those ship in a release.

• docs/using/emulator-streaming.md: a new Picking a RetroArch core section covering the retroarch:<core> shorthand and the block form, what each broker tier does at launch, the experimental_cores opt-in, what happens on a broker too old to take a core, and that states stay with the core that wrote them while in-game saves carry over. It links to the broker's core guide for the tier lists. The pooling rule and the Save states section mention core too, and the setup example shows the shorthand.
• docs/reference/configuration-file.md: core and experimental_cores rows in the container table, the shorthand and new override keys in the platforms sentence, both keys in the pooling rule, and a shorthand line in the example.

The second commit is the humanize-docs pass over the new prose. uv run mkdocs build --strict and scripts.check_redirects pass. Prettier (trunk's pinned 3.5.2 and config) is clean. trunk check couldn't run its SARIF parsers locally, because a shared library is missing on my machine, so CI is the real lint gate here.

AI assistance: this PR was written by Claude Code, under my direction and review.