document next

[xhliu]

next?: StatefulNext<T>[] | StatefulNext<T>,

https://docs.scrypt.io/how-to-deploy-and-call-a-contract/#methodcalloptions

Explain where, when and how is this used.

• used in txBuilder only, anywhere else?
• can be omitted in the option and created in txBuilder (add and link to example, already exist in boilerplate?)

[yusufidimaina9989]

Done. https://github.com/sCrypt-Inc/scryptTS-docs/pull/226