docs(library-docs): document supported Doxyfile settings

[devin-ai-integration]

Summary

For C++ library docs, Fern no longer hands a customer's Doxyfile to doxygen as-is. It now reads only four settings from it (ALIASES, PREDEFINED, FILE_PATTERNS, OPTIMIZE_OUTPUT_FOR_C) into its own doxygen configuration and ignores everything else. That makes doxygen configuration predictable and safe, but it also means settings that used to work — notably INPUT and @INCLUDE — are now silently dropped, and invalid values for the four supported settings fail the docs publish. Authors configuring config.doxyfile need the supported surface spelled out, and the page's existing claim that Doxyfile input directives widen the source root is now wrong.

Implements docs for: fix(fdr): sanitize user-provided doxyfile settings through an allowlist (fern-api/fern-platform#14062)

Pages changed

• fern/products/docs/pages/api-references/library-docs.mdx
  • config.doxyfile in the configuration reference now lists the four honored settings, states that everything else (including INPUT, INPUT_FILTER, GENERATE_XML, @INCLUDE, @INCLUDE_PATH) is ignored, and describes what makes a value invalid enough to fail publishing (spaces/shell metacharacters in FILE_PATTERNS, non-YES/NO booleans, malformed alias definitions, trailing backslashes, $(VAR) expansion) plus the 262,144-character content limit.
  • Corrected the C++ source-root note, which said the effective scope was path combined with the Doxyfile's input entries. The parser uses the input.path directory as the source root; Doxyfile input directives have no effect.
• fern/products/docs/pages/changelog/2026-08-20.mdx — new entry for the behavior change, since previously-honored settings stop taking effect.

Link to Devin session: https://app.devin.ai/sessions/67938d5b35e7404a93790995cf9f21b8

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1787245687-doxyfile-allowlist.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/api-references/library-reference
• learn/docs/changelog/2026/8/20