describe: warn user that describing multiple commits is experimental

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.

CHANGELOG.md

@@ -103,7 +103,7 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 * Added revset functions `author_date` and `committer_date`.
 
-* `jj describe` can now update the description of multiple commits.
+* `jj describe` can now update the description of multiple commits (experimental).
 
 ### Fixed bugs
 

cli/src/description_util.rs

@@ -60,8 +60,13 @@ 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: Describing multiple commits at once is currently
+        JJ: **experimental**.
+        JJ: - The text you enter will be lost on a syntax error.
+        JJ: - The syntax 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

@@ -225,6 +225,11 @@ fn test_describe_multiple_commits() {
         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: Describing multiple commits at once is currently
+    JJ: **experimental**.
+    JJ: - The text you enter will be lost on a syntax error.
+    JJ: - The syntax may change in the future.
+
     JJ: describe 8d650510daad -------
 
     JJ: describe 41659b846096 -------
@@ -238,6 +243,13 @@ fn test_describe_multiple_commits() {
         &edit_script,
         indoc! {"
             write
+            JJ: Enter or edit commit descriptions after the `JJ: describe` lines.
+
+            JJ: Warning: Describing multiple commits at once is currently
+            JJ: **experimental**.
+            JJ: - The text you enter will be lost on a syntax error.
+            JJ: - The syntax may change in the future.
+
             JJ: describe 8d650510daad -------
             description from editor of @-