docs: impr process output descr

Author: antongolub

docs/process-output.md

@@ -8,7 +8,7 @@ const o = await p     // ProcessOutput
 ```
 
 ```ts
-interface ProcessOutput {
+interface ProcessOutput extends Error {
   // Exit code of the process: 0 for success, non-zero for failure
   exitCode: number
 

docs/process-promise.md

@@ -1,6 +1,6 @@
 # Process Promise
 
-The `$` returns a `ProcessPromise` instance. When resolved, it becomes a [`ProcessOutput`](./process-output.md).
+The `$` returns a `ProcessPromise` instance, which inherits native `Promise`. When resolved, it becomes a [`ProcessOutput`](./process-output.md).
 
 ```js
 const p = $`command` // ProcessPromise