doc: fix webstream close reject state state doc

The `closed` promise may reject. Document the states where it rejects
based on the spec.

Fixes: #41858
Co-Authored-By: Mattias Buelens <mattias@buelens.com>

doc/api/webstreams.md

@@ -425,7 +425,8 @@ added: v16.5.0
 -->
 
 * Type: {Promise} Fulfilled with `undefined` when the associated
-  {ReadableStream} is closed or this reader's lock is released.
+  {ReadableStream} is closed or rejected if the stream errors or the reader's
+  lock is released before the stream finishes closing.
 
 #### `readableStreamDefaultReader.read()`
 
@@ -552,7 +553,8 @@ added: v16.5.0
 -->
 
 * Type: {Promise} Fulfilled with `undefined` when the associated
-  {ReadableStream} is closed or this reader's lock is released.
+  {ReadableStream} is closed or rejected if the stream errors or the reader's
+  lock is released before the stream finishes closing.
 
 #### `readableStreamBYOBReader.read(view)`