Document use of allow_concurrent_runs with long-running tasks (zed-…

…industries#20539)

docs/src/tasks.md

@@ -50,6 +50,10 @@ There are two actions that drive the workflow of using tasks: `task: spawn` and
 `task: spawn` opens a modal with all available tasks in the current file.
 `task: rerun` reruns the most-recently spawned task. You can also rerun tasks from task modal.
 
+By default, rerunning tasks reuses the same terminal (due to the `"use_new_terminal": false` default) but waits for the previous task to finish before start (due to the `"allow_concurrent_runs": false` default).
+
+Keep `"use_new_terminal": false` and set `"allow_concurrent_runs": true` to allow cancelling previous tasks on rerun.
+
 ## Task templates
 
 Tasks can be defined: