describe: warn user of limitations when describing multiple commits

I think it might be nice to have this in the upcoming release, but I'd
like to warn people that their changes will be lost if they aren't
careful, and to not rely on the syntax being fixed just yet.

cli/src/description_util.rs

@@ -60,8 +60,11 @@ pub fn edit_multiple_descriptions(
     let mut bulk_message = String::new();
 
     bulk_message.push_str(indoc! {r#"
-    JJ: Enter or edit commit descriptions after the `JJ: describe` lines.
-    
+        JJ: Enter or edit commit descriptions after the `JJ: describe` lines.
+        JJ: Warning:
+        JJ: - The text you enter will be lost on a syntax error.
+        JJ: - The syntax of separate lines may change in the future.
+
     "#});
     for (commit_id, temp_commit) in commits.iter() {
         let commit_hash = short_commit_hash(commit_id);

cli/tests/test_describe_command.rs

@@ -224,6 +224,9 @@ fn test_describe_multiple_commits() {
     insta::assert_snapshot!(
         std::fs::read_to_string(test_env.env_root().join("editor0")).unwrap(), @r###"
     JJ: Enter or edit commit descriptions after the `JJ: describe` lines.
+    JJ: Warning:
+    JJ: - The text you enter will be lost on a syntax error.
+    JJ: - The syntax of separate lines may change in the future.
 
     JJ: describe 8d650510daad -------
 
@@ -238,6 +241,10 @@ fn test_describe_multiple_commits() {
         &edit_script,
         indoc! {"
             write
+            JJ: Enter or edit commit descriptions after the `JJ: describe` lines.
+
+            JJ: More header tests. Library tests verify parsing in other situations.
+
             JJ: describe 8d650510daad -------
             description from editor of @-