docs(js): Update nextjs docs on tunnelRoute option

[chargome]

documents getsentry/sentry-javascript#16626

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
sentry-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 8, 2025 8:04am

1 Skipped Deployment

Name	Status	Preview	Comments	Updated (UTC)
develop-docs	⬜️ Ignored (Inspect)	Visit Preview		Jul 8, 2025 8:04am

[codecov]

Bundle Report

Changes will decrease total bundle size by 15 bytes (-0.0%) ⬇️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
sentry-docs-server-cjs	10.92MB	-9 bytes (-0.0%) ⬇️
sentry-docs-client-array-push	9.77MB	-6 bytes (-0.0%) ⬇️

Affected Assets, Files, and Routes:

view changes for bundle: sentry-docs-server-cjs

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
1729.js	-3 bytes	1.64MB	-0.0%
../instrumentation.js	-3 bytes	973.36kB	-0.0%
9523.js	-3 bytes	949.31kB	-0.0%

view changes for bundle: sentry-docs-client-array-push

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
static/chunks/pages/_app-*.js	-3 bytes	873.24kB	-0.0%
static/chunks/7750-*.js	-3 bytes	401.83kB	-0.0%
static/uxu7tziPvylLsLWH413T9/_buildManifest.js (New)	616 bytes	616 bytes	100.0% 🚀
static/uxu7tziPvylLsLWH413T9/_ssgManifest.js (New)	77 bytes	77 bytes	100.0% 🚀
static/P9cNiAJohrKD_X6y6Fl94/_buildManifest.js (Deleted)	-616 bytes	0 bytes	-100.0% 🗑️
static/P9cNiAJohrKD_X6y6Fl94/_ssgManifest.js (Deleted)	-77 bytes	0 bytes	-100.0% 🗑️