Validates documentation references in usage-rules-managed markdown files by driving ex_doc's own autolink pipeline over them.

This is the engine behind mix usage_rules.validate. Each file is parsed and autolinked exactly like an extra page in a docs build — through ex_doc's extras builder and its formatter autolink pass — and the warnings are ex_doc's own, printed with file/line information: the same warnings you would get from a HexDocs build over the same content.

What ex_doc checks

• function references with arity in inline code spans (Module.function/arity, :erlang.function/arity) — including @doc false / @moduledoc false targets, which are reported as "hidden or private"
• hidden modules mentioned in inline code spans
• explicit reference links in backtick form: [`Mod`](`Mod`) and [`mix task`](`mix task`)
• relative file links between the validated files (links to files that are not part of the validated set do not resolve and are reported)

What ex_doc does not check

ex_doc deliberately skips fenced code blocks when autolinking, and it stays silent about plain code span mentions that never resolve, such as mix task names and bare undefined module names — the same silence you get inside moduledocs. Validation warnings are exactly a docs build's warnings, nothing more and nothing less.

ex_doc must be compiled and available. When it is not, validation fails with an actionable error (see ensure_ex_doc!/1).