Correct ISP and datacenter IP stability in managed auth proxy docs

[courtney918]

Summary

The Custom Proxy section of auth/configuration.mdx said ISP and datacenter proxies give a stable exit IP within a single session but no guarantee across sessions. That contradicts the proxy docs and the implementation:

• ISP: each ISP proxy is pinned to one endpoint when created, so its exit IP persists across sessions (proxies/isp.mdx). The login, health checks and reauths all use the same IP.
• Datacenter: exit IPs rotate per request, even within a session (proxies/datacenter.mdx).

Changes:

• Split the combined ISP/datacenter bullet into two accurate bullets. The ISP bullet notes that each ISP proxy has its own IP, so users should reuse one proxy.
• Reword the custom proxy bullet so it no longer implies custom is the only static-IP option. It's now positioned as the choice when you need to control the egress IP yourself.

Docs-only change; not previewed locally.

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