cli util gc: document the fact that it does not abandon operations

This UI will probably change eventually, but let's document it for now.

cli/src/commands/util/gc.rs

@@ -24,6 +24,9 @@ use crate::command_error::CommandError;
 use crate::ui::Ui;
 
 /// Run backend-dependent garbage collection.
+///
+/// To garbage-collect old operations and the commits/objects referenced by
+/// then, run `jj op abandon ..<some old operation>` before `jj util gc`.
 #[derive(clap::Args, Clone, Debug)]
 pub struct UtilGcArgs {
     /// Time threshold

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

@@ -2264,7 +2264,9 @@ echo "args: $@"
 
 ## `jj util gc`
 
-Run backend-dependent garbage collection
+Run backend-dependent garbage collection.
+
+To garbage-collect old operations and the commits/objects referenced by then, run `jj op abandon ..<some old operation>` before `jj util gc`.
 
 **Usage:** `jj util gc [OPTIONS]`