docs(react): document the _tagIds id-tagging option

[moss-bryophyta]

What

Documents the opt-in _tagIds option in the Next.js withGTConfig reference. It shipped in gt-next@11.1.6 (published 2026-08-10) via gt#2017 and was not documented anywhere in the docs.

Why it needs care

_tagIds injects markup, so the docs have to be precise about when. I wrote this against the implementation in renderPreparedT.shared.ts rather than the changelog, which surfaced two cases the changelog summary does not spell out:

• Component roots also get wrapped. The wrapper check is typeof rendered.type === 'string', so it matches host elements only. A <T> wrapping a component root takes a span just like bare text and fragments. The changelog mentions only "bare text or a fragment."
• Output that renders nothing is deliberately left untouched — null, undefined, booleans, '', arrays whose entries all render nothing, and empty fragments. rendersNothing() exists specifically so an empty <span data-_gt-hash> never appears and changes :empty/child structure. 0 and NaN do render and are tagged normally.

Also documented: the literal-true requirement (isIdTaggingEnabled() is === true, and there is a test asserting truthy-but-malformed values stay off), no effect on gt()/useGT() strings, and the React Native skip.

Verification

• All five validators pass. The reference-link validator caught a bare `<T>` in the options table on the first run; fixed by linking it to /docs/react/reference/components/t.
• Confirmed gt-next@11.1.6 is the current published version on npm.
• Confirmed no other doc page or open PR covers _tagIds.

Reviewers

@logflash authored the feature, so flagging the component-root and renders-nothing cases for confirmation — I derived them from reading the source, not from the changelog.