Clarify what savvy won't support

[yutannihilation]

Never:

• data frame input (treat it as a simple list)
• data frame output (return a list and use tibble())

Unlikely:

• Custom error class. There's no handy API for creating and raising a custom error. R_errorcall() doesn't provide a way to set custom classes. Actually, rlang's C implementation says "Probably should be implemented at R level."
• ALTREP
• matrix. There are too many options. But, how to deal with matrix (or dim) should be documented

[yutannihilation]

• Clarify that it's highly unsafe to convert protected types (e.g. Owned...Sexp and FunctionCallResult) to Sexp is highly unsafe because it loses protection, and it's the user's responsibility to protect it.