doc: add missing types

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

doc/api/stream.md

@@ -499,6 +499,8 @@ 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`.
@@ -518,6 +520,8 @@ the [`'finish'`][] event has been emitted.
 added: v12.3.0
 -->
 
+* {boolean}
+
 Getter for the property `objectMode` of a given `Writable` stream.
 
 ##### writable.write(chunk[, encoding][, callback])
@@ -1112,6 +1116,8 @@ the status of the `highWaterMark`.
 added: v12.3.0
 -->
 
+* {boolean}
+
 Getter for the property `objectMode` of a given `Readable` stream.
 
 ##### readable.resume()