Update transports.md

[janvernieuwe]

Update serializer handler docs

Q	A
Type	docs update
BC Break	no
Fixed issues	n/a

Q: Is it an idea to set \stdClass as a default for return type?

[veewee]

Doesn't feel good to make it the default type.
We might add a method that tells the transport not to expect any output?
In other cases, a DTO seems like tha way to go. If you really want to use stdClass, then that is perfectly possible to manually determine as a return type.