tests.nixpkgs-check-by-name: Move interface description into code

This would be duplicated otherwise

pkgs/test/nixpkgs-check-by-name/README.md

@@ -4,28 +4,14 @@ This directory implements a program to check the [validity](#validity-checks) of
 It is being used by [this GitHub Actions workflow](../../../.github/workflows/check-by-name.yml).
 This is part of the implementation of [RFC 140](https://github.com/NixOS/rfcs/pull/140).
 
-## API
+## Interface
 
-This API may be changed over time if the CI workflow making use of it is adjusted to deal with the change appropriately.
+The interface of the tool is shown with `--help`:
+```
+cargo run -- --help
+```
 
-- Command line: `nixpkgs-check-by-name [--base <BASE_NIXPKGS>] <NIXPKGS>`
-- Arguments:
-  - `<NIXPKGS>`:
-    The path to the Nixpkgs to check.
-    For PRs, this should be set to a checkout of the PR branch.
-  - `<BASE_NIXPKGS>`:
-    The path to the Nixpkgs to use as the [ratchet check](#ratchet-checks) base.
-    For PRs, this should be set to a checkout of the PRs base branch.
-
-    If not specified, no ratchet checks will be performed.
-    However, this flag will become required once CI uses it.
-- Exit code:
-  - `0`: If the [validation](#validity-checks) is successful
-  - `1`: If the [validation](#validity-checks) is not successful
-  - `2`: If an unexpected I/O error occurs
-- Standard error:
-  - Informative messages
-  - Detected problems if validation is not successful
+The interface may be changed over time only if the CI workflow making use of it is adjusted to deal with the change appropriately.
 
 ## Validity checks
 

pkgs/test/nixpkgs-check-by-name/src/main.rs

@@ -17,8 +17,20 @@ use std::path::{Path, PathBuf};
 use std::process::ExitCode;
 
 /// Program to check the validity of pkgs/by-name
+///
+/// This CLI interface may be changed over time if the CI workflow making use of
+/// it is adjusted to deal with the change appropriately.
+///
+/// Exit code:
+/// - `0`: If the validation is successful
+/// - `1`: If the validation is not successful
+/// - `2`: If an unexpected I/O error occurs
+///
+/// Standard error:
+/// - Informative messages
+/// - Detected problems if validation is not successful
 #[derive(Parser, Debug)]
-#[command(about)]
+#[command(about, verbatim_doc_comment)]
 pub struct Args {
     /// Path to the main Nixpkgs to check.
     /// For PRs, this should be set to a checkout of the PR branch.