DSLX Documentation: send/recv and interpreter options

It would be helpful if [DSLX Documentation](https://google.github.io/xls/dslx_reference/) contained information about:
- all [builtins](https://github.com/google/xls/blob/6563263c575f0dc57c7346b9496cdc01581a77b5/xls/dslx/frontend/builtins_metadata.cc#L86). Currently send/recv functions are not documented and I rely on this file to check the correct signature while learning the language.
- Interpreter CLI arguments, currently only listed in [the source code](https://github.com/google/xls/blob/6563263c575f0dc57c7346b9496cdc01581a77b5/xls/dslx/interpreter_main.cc#L44)

I would be grateful, if you could add these 2 items to the docs.