docs CLI reference: add a warning that it is experimental

There are many minor bugs that are difficult to fix in the short-term.

See also the commit message for a197409

cli/tests/cli-reference@.md.snap

@@ -2,6 +2,14 @@
 source: cli/tests/test_generate_md_cli_help.rs
 description: "AUTO-GENERATED FILE, DO NOT EDIT. This cli reference is generated as an `insta` snapshot. MkDocs follows they symlink from docs/cli-reference.md to the snap. Unfortunately, `insta` unavoidably creates this header. Luckily, MkDocs ignores the header since it has the same format as Markdown headers. TODO: MkDocs may fail on Windows if symlinks are not enabled in the OS settings"
 ---
+
+!!! warning
+
+    This CLI reference is experimental. It is automatically generated, but
+    does not match the `jj help` output exactly.
+
+    <!-- See also https://github.com/ConnorGray/clap-markdown/issues -->
+
 # Command-Line Help for `jj`
 
 This document contains the help content for the `jj` command-line program.

cli/tests/test_generate_md_cli_help.rs

@@ -18,10 +18,22 @@ use crate::common::TestEnvironment;
 
 pub mod common;
 
+const PREAMBLE: &str = r#"
+!!! warning
+
+    This CLI reference is experimental. It is automatically generated, but
+    does not match the `jj help` output exactly.
+
+    <!-- See also https://github.com/ConnorGray/clap-markdown/issues -->
+
+"#;
+
 #[test]
 fn test_generate_markdown_docs_in_docs_dir() {
     let test_env = TestEnvironment::default();
-    let markdown_help = test_env.jj_cmd_success(test_env.env_root(), &["util", "markdown-help"]);
+    let mut markdown_help = PREAMBLE.to_string();
+    markdown_help
+        .push_str(&test_env.jj_cmd_success(test_env.env_root(), &["util", "markdown-help"]));
     // Validate partial snapshot, redacting any lines nested 2+ indent levels.
     insta::with_settings!({
         snapshot_path => ".",