Convert API definitions section headings to sentence case

[devin-ai-integration]

Convert API definitions section headings to sentence case

Summary

Converted all headings (H2-H6) in the API definitions section to sentence case while preserving proper nouns and technical terms. Also updated the Vale Headings.yml configuration with 29 new exceptions for API-related technical terms.

Changes made:

• Vale configuration: Added exceptions for OpenAPI, AsyncAPI, Fern Definition, TLS, JWT, OAuth2, Protocol Buffers, Kubernetes, FastAPI, and other technical terms
• Heading updates: Changed ~150+ headings across gRPC, OpenAPI, AsyncAPI, OpenRPC, and Fern Definition documentation from title case to sentence case
• Scope: API definitions section only (fern/products/api-def/)

Examples:

• "API Key Authentication" → "API key authentication"
• "Bearer Token Authentication" → "Bearer token authentication"
• "Protocol Buffer Best Practices" → "Protocol buffer best practices"
• "Multiple Base URLs for a single API" → "Multiple base URLs for a single API"

Review & Testing Checklist for Human

• Verify Vale exceptions are comprehensive: Check that all technical terms and proper nouns are correctly preserved (especially look at gRPC, OpenAPI, AsyncAPI headings)
• Spot check sentence case application: Review a few files to ensure headings read naturally and no errors were introduced by the automated sed scripts
• Run Vale locally: Verify that Vale passes on the changed files and that the new exceptions work as intended
• Check for missed headings: Search for any remaining title case headings in the API definitions section that should have been converted

Notes

• Used sed scripts for bulk changes, so there's a small risk of inconsistent application
• Some trailing whitespace was added/removed in the process (cosmetic only)
• Navigation titles in api-def.yml were already mostly in sentence case, so minimal changes there
• Could not run Vale locally to verify, so CI checks will be important

Session: https://app.devin.ai/sessions/ba3a1ee1862744b0b5751f786a6ace2e
Requested by: Devin Logan (@devalog)

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