doc: fix the description of 'close' event

PR-URL: https://github.com/nodejs/node/pull/15800
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

doc/api/fs.md

@@ -182,7 +182,7 @@ added: v0.1.93
 
 * `fd` {integer} Integer file descriptor used by the ReadStream.
 
-Emitted when the ReadStream's file is opened.
+Emitted when the `ReadStream`'s underlying file descriptor has been closed.
 
 ### readStream.bytesRead
 <!-- YAML
@@ -296,7 +296,7 @@ added: v0.1.93
 
 * `fd` {integer} Integer file descriptor used by the WriteStream.
 
-Emitted when the WriteStream's file is opened.
+Emitted when the `WriteStream`'s underlying file descriptor has been closed.
 
 ### writeStream.bytesWritten
 <!-- YAML