doc: add missing types

Document the types returned by the `readable.readableObjectMode`,
`writable.writableLength`, and `writable.writableObjectMode` getters.

PR-URL: #28623
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/stream.md

@@ -504,6 +504,8 @@ Return the value of `highWaterMark` passed when constructing this
 added: v9.4.0
 -->
 
+* {number}
+
 This property contains the number of bytes (or objects) in the queue
 ready to be written. The value provides introspection data regarding
 the status of the `highWaterMark`.
@@ -513,6 +515,8 @@ the status of the `highWaterMark`.
 added: v12.3.0
 -->
 
+* {boolean}
+
 Getter for the property `objectMode` of a given `Writable` stream.
 
 ##### writable.write(chunk[, encoding][, callback])
@@ -1107,6 +1111,8 @@ the status of the `highWaterMark`.
 added: v12.3.0
 -->
 
+* {boolean}
+
 Getter for the property `objectMode` of a given `Readable` stream.
 
 ##### readable.resume()