docs: lead with the third option, and correct a false claim about Base

[sotashimozono]

First release candidate. Two changes, both to what the package says.

The headline was the weakest claim available

The README opened on "every public name is documented or declared" — which is the half the
ecosystem already covers. Docs.undocumented_names has been public API in Base since 1.11, and
Aqua.test_undocumented_names ships it as a test. Opening there invites exactly one reaction.

It now opens on the part neither can express — the third option — and states plainly why the
declaration cannot live in Aqua: a mark is written in src/, and Aqua is a test-only dependency.
It can own the check; it cannot own the declaration.

A claim about Base that was simply wrong

A comment in src/audit.jl and another in test/test_audit.jl asserted that Docs.hasdoc is
not public Base API. It is — public, and exported from Base.Docs. Corrected, and replaced
with the actual reason the per-name form is used here:

names(Down)                    = [:Down, :own_undoc, :undoc_up, :up]
Docs.undocumented_names(Down)  = [:Down, :own_undoc, :undoc_up]   # a dependency's gap, as yours
audit(Down).unaccounted        = [:own_undoc]                     # only what Down owns
audit(Down).foreign            = [:undoc_up, :up]

Docs.undocumented_names reports a re-exported name's missing docstring as though it were this
module's to fix. audit has to keep foreign apart from a genuine gap, which the set form
cannot do.

Also

Drops the README's measured five-package table. Those counts age the moment any of those
packages gains a docstring, and nothing checks them.

Version

0.1.0 → 0.1.1. No API change; docs and comments only.

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📚 Docs preview: https://codes.sota-shimozono.com/ExperimentalAPI.jl/previews/PR4/

(updates on each push to this PR)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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