doc: undocumented properties are mentioned in the stream doc

[vsemozhetbyt]

writable.writableBuffer and readable.readableBuffer are mentioned https://nodejs.org/api/stream.html#stream_buffering but are undocumented in the appropriate sections.

Not sure if this need to be addressed. Please close if this is OK.

[lpinca]

I'm not sure if it makes sense to document them because the getters return an internal type (BufferList) which should not be public.

[lpinca]

cc: @nodejs/streams

[lpinca]

I guess the whole point for adding the getters was to give users the ability to easily access the BufferList so ¯_(ツ)_/¯.

[reergymerej]

If nothing else, the language is confusing.

internal buffer that can be retrieved using writable.writableBuffer

but it returns a BufferList.