Add note about graceful termination (#562)

index.d.ts

@@ -460,7 +460,7 @@ export type ExecaChildPromise<StdoutStderrType extends StdoutStderrAll> = {
 	): Promise<ExecaReturnValue<StdoutStderrType> | ResultType>;
 
 	/**
-	Same as the original [`child_process#kill()`](https://nodejs.org/api/child_process.html#child_process_subprocess_kill_signal), except if `signal` is `SIGTERM` (the default value) and the child process is not terminated after 5 seconds, force it by sending `SIGKILL`.
+	Same as the original [`child_process#kill()`](https://nodejs.org/api/child_process.html#child_process_subprocess_kill_signal), except if `signal` is `SIGTERM` (the default value) and the child process is not terminated after 5 seconds, force it by sending `SIGKILL`. Note that this graceful termination does not work on Windows, because Windows [doesn't support signals](https://nodejs.org/api/process.html#process_signal_events) (`SIGKILL` and `SIGTERM` has the same effect of force-killing the process immediately.) If you want to achieve graceful termination on Windows, you have to use other means, such as [`taskkill`](https://github.com/sindresorhus/taskkill).
 	*/
 	kill(signal?: string, options?: KillOptions): void;
 

readme.md

@@ -287,6 +287,8 @@ The return value of all [asynchronous methods](#methods) is both:
 
 Same as the original [`child_process#kill()`](https://nodejs.org/api/child_process.html#child_process_subprocess_kill_signal) except: if `signal` is `SIGTERM` (the default value) and the child process is not terminated after 5 seconds, force it by sending `SIGKILL`.
 
+Note that this graceful termination does not work on Windows, because Windows [doesn't support signals](https://nodejs.org/api/process.html#process_signal_events) (`SIGKILL` and `SIGTERM` has the same effect of force-killing the process immediately.) If you want to achieve graceful termination on Windows, you have to use other means, such as [`taskkill`](https://github.com/sindresorhus/taskkill).
+
 ##### options.forceKillAfterTimeout
 
 Type: `number | false`\