chore(documentation): correct exec advice (#738)

Author: james-rl

llms.txt

@@ -23,8 +23,10 @@
 - Prefer SDK methods (`sdk.devbox`, `sdk.blueprint`, etc.) for object-oriented patterns and convenience
 - For resources without SDK coverage (e.g., secrets, benchmarks), use `sdk.api.*` as a fallback
 - Use `sdk.devbox.create()` to create devboxes; they auto-await readiness
-- Execute commands with `devbox.cmd.exec('command')`, not raw API calls
-- Always call `devbox.shutdown()` to clean up resources
+- Use `devbox.cmd.exec('command')` for most commands—blocks until completion, returns `ExecutionResult` with stdout/stderr
+- Use `devbox.cmd.execAsync('command')` for long-running or background processes (servers, watchers)—returns immediately with `Execution` handle to check status, get result, or kill
+- Both `exec` and `execAsync` support streaming callbacks (`stdout`, `stderr`, `output`) for real-time output
+- Call `devbox.shutdown()` to clean up resources that are no longer in use.
 - In example files, focus on the `recipe` function body for SDK usage patterns; ignore test harness files in the examples folder (`_harness.ts`, `registry.ts`, `types.ts`)
 
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