PR #25863: docs(proxy): document enable_post_custom_auth_checks for custom auth

• Repository: BerriAI/litellm
• Author: shaun0927
• State: open | merged: False
• Link: https://github.com/BerriAI/litellm/pull/25863

Description (problem / solution / changelog)

Relevant issues

• Closes #25862

Pre-Submission checklist

Please complete all items before asking a LiteLLM maintainer to review your PR

• I have Added testing in the tests/test_litellm/ directory
• My PR passes all unit tests on make test-unit
• My PR's scope is as isolated as possible, it only solves 1 specific problem
• I have requested a Greptile review by commenting @greptileai and received a Confidence Score of at least 4/5 before requesting a maintainer review

Notes:

• This is a docs-only PR. It does not change runtime behavior.
• I did not mark the testing items complete because I only updated proxy documentation.

Summary

The current proxy docs mention custom_auth_run_common_checks, but they do not document the outer enable_post_custom_auth_checks gate that actually enables LiteLLM's post-custom-auth validation flow.

That creates a docs gap for operators using custom auth:

• the code and tests make the opt-in behavior explicit
• the published docs only mention the inner custom_auth_run_common_checks flag
• readers can incorrectly assume the inner flag is sufficient on its own

Type

📖 Documentation

Changes

1. Add enable_post_custom_auth_checks to docs/my-website/docs/proxy/config_settings.md

   • documents the flag
   • documents the default (false)
   • explains that the default is performance-oriented

2. Add a short explanation to docs/my-website/docs/proxy/custom_auth.md

   • shows how enable_post_custom_auth_checks interacts with custom_auth_run_common_checks
   • explains which built-in checks run when post-custom-auth checks are enabled
   • makes it explicit that custom_auth_run_common_checks only has an effect inside the post-custom-auth flow

Screenshots / Proof of Fix

Docs-only change.

Proof from current code/docs state before this PR:

• litellm/proxy/auth/user_api_key_auth.py gates _run_post_custom_auth_checks(...) behind enable_post_custom_auth_checks
• docs/my-website/docs/proxy/config_settings.md documented custom_auth_run_common_checks but not enable_post_custom_auth_checks

After this PR:

• both docs pages describe the opt-in flag and its interaction with the common-check flag
• no runtime behavior changes

Changed files

• docs/my-website/docs/proxy/config_settings.md (modified, +1/-0)
• docs/my-website/docs/proxy/custom_auth.md (modified, +24/-0)