docs(config_settings): document fallback generalizations env vars

[devin-ai-integration]

Summary

BerriAI/litellm#36422 moves the routing fallback_generalizations rules out of the model cost map and into their own file, fetched from their own URL with the bundled copy as backup. That adds two env vars, LITELLM_FALLBACK_GENERALIZATIONS_URL and LITELLM_LOCAL_FALLBACK_GENERALIZATIONS, which follow the same remote plus local backup shape already documented for the Anthropic beta headers and blog posts

This adds their rows to the environment variables reference table. tests/documentation_tests/test_env_keys.py in the litellm repo fails until they are documented here, so the code PR stays red until this lands

Link to Devin session: https://app.devin.ai/sessions/b8d7e177e9144ff6a370b11e7e0e7c9c
Requested by: @mateo-berri

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Project	Deployment	Actions	Updated (UTC)
litellm	Ready	Preview	Aug 10, 2026 5:05pm

[devin-ai-integration]

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