Update protocol docs w.r.t. call

[pnwamk]

Since GaloisInc/cryptol#944 call is now more flexible (any expression can appear in operator position), we need to document this in the protocol.

[pnwamk]

(Also, mark call as deprecated in favor of just using evaluate_expression, since the latter is purely more expressive/useful than the former).

[weaversa]

call should be deprecated?

[pnwamk]

After chatting more with @david-christiansen it seems the suggestion to deprecate call was more of a comment on the JSON-RPC API and backend that I misinterpreted (sorry for any confusion). If the call method on the CryptolConnection Python class is useful we can absolutely keep it (and internally it will just be a thin wrapper around the more generic expression evaluation feature).

[weaversa]

evaluate_expression is certainly more expressive, but also more prone to error. One nice thing about call is that it takes JSON arguments, so passing a result from a previous invocation of a Cryptol function is trivial.