Add documentation about exported WASI functions

[mathetake]

Given that Proxy-Wasm is a sort of "extension" of WASI, we should have the document about which functions in WASI should be exported to WasmVMs in this specification. That would be helpful for both SDK developers and users.

Actually, some of WASI functions are already available in cpp-host, though some of them just return __WASI_ESUCCESS and are virtually unimplemented.