doc: clarify fs.ReadStream and fs.WriteStream are not constructable

Add explicit wording that fs.ReadStream and fs.WriteStream should not
be constructed directly, matching the existing pattern used by fs.Stats
("not to be created directly using the new keyword"). The factory
functions fs.createReadStream() and fs.createWriteStream() are the
supported API.

Fixes: #40546

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kovan

doc/api/fs.md

@@ -7147,8 +7147,8 @@ added: v0.1.93
 
 * Extends: {stream.Readable}
 
-Instances of {fs.ReadStream} are created and returned using the
-[`fs.createReadStream()`][] function.
+`fs.ReadStream` is not to be constructed directly; use
+[`fs.createReadStream()`][] instead.
 
 #### Event: `'close'`
 
@@ -7918,8 +7918,8 @@ added: v0.1.93
 
 * Extends {stream.Writable}
 
-Instances of {fs.WriteStream} are created and returned using the
-[`fs.createWriteStream()`][] function.
+`fs.WriteStream` is not to be constructed directly; use
+[`fs.createWriteStream()`][] instead.
 
 #### Event: `'close'`