document that jj util completion defaults to bash

it's somewhat confusing to me that the `--bash` flag exists at all, since it does nothing - maybe it makes sense to give a hard error? or just remove the flag?
but in any case, it seems good to document the existing behavior.

cli/src/commands/util.rs

@@ -36,7 +36,7 @@ pub(crate) enum UtilCommand {
 /// Print a command-line-completion script
 #[derive(clap::Args, Clone, Debug)]
 pub(crate) struct UtilCompletionArgs {
-    /// Print a completion script for Bash
+    /// Print a completion script for Bash (default)
     ///
     /// Apply it by running this:
     ///

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

@@ -1712,7 +1712,7 @@ Print a command-line-completion script
 
 ###### **Options:**
 
-* `--bash` — Print a completion script for Bash
+* `--bash` — Print a completion script for Bash (default)
 
   Possible values: `true`, `false`