docs: streaming while self-hosting must disable buffering

[leerob]

Closes #54667

[coltonehrman]

@leerob This is awesome! We actually ran into the same issue, and instead used the X-Accel-Buffering header to disable buffering specifically for Next.js responses vs turning off buffering completely (which isn't recommended by nginx).

  async headers() {
    return [
      {
        source: "/:path*{/}?",
        headers: [
          {
            key: "x-accel-buffering",
            value: "no",
          },
        ],
      },
    ];
  },

[leerob]

Thank you @coltonehrman, updated: #61010