docs: clear all ex_doc warnings

[Taure]

rebar3 ex_doc emitted 123 warnings. Now it emits none.

122 of them: rebar_state:t/0

warning: spec references type "rebar_state:t/0" but it is undefined or private
  └─ (rebar3_nova) src/rebar3_nova_serve.erl:26

One per reference, across all 16 modules. Every provider spec names rebar_state:t():

-spec init(rebar_state:t()) -> {ok, rebar_state:t()}.

rebar3 is the host application, not a dependency, so its modules are absent from the doc build and ex_doc cannot resolve the type. That will not change - rebar3 ships as an escript with no ebin directory to point deps at. The specs are correct; the warning is unresolvable noise.

What I tried first

skip_code_autolink_to looked like the precise fix - suppress by type name. It does nothing here: the spec-autolink path never consults it, and rebar3_ex_doc reports it as unknown ex_doc option. Warnings stayed at 122.

The option that works is skip_undefined_reference_warnings_on, and two things about it are worth stating plainly:

• It matches on module or file, not on the referenced type (ExDoc.Autolink.maybe_warn/4 tests config.id, config.module_id and the file path). So it cannot be narrowed to rebar_state:t/0 - the granularity does not exist.
• All 16 modules is every module, so for module docs this is the whole warning class, not a targeted allowlist. I would rather say that than let the list imply otherwise.

What redeems it: extras are not covered. I verified that by pointing README.md at a non-existent file and confirming it still warned:

warning: documentation references file "does-not-exist.md" but it does not exist
  └─ README.md: (file)

So broken links in README.md and LICENSE keep surfacing - which for a plugin is where the docs actually live, main being readme.

The known wart: a newly added module will not be in the list and will start warning again. It is self-announcing, and the comment in rebar.config explains why the list exists.

The 123rd

README.md:2 had a stray ===== setext underline sitting under an ATX # heading. Removed.

Checks

ex_doc 0 warnings, fmt --check, xref, ct 28/28 all clean.