docs: Document WASI support level

docs/WASI.md

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+SLEdge only implemented a subset of the WASI syscall interface 
+
+## Arguments
+
+The WASI calls `args_sizes_get` and `args_get` are supported. HTTP query parameters are captured and passed as arguments.
+
+## Environment Variables
+
+The WASI calls `environ_get` and `environ_sizes_get` are supported, but mostly unused. The current behavior is to to pass the runtime's environment variables into the sandbox. This is likely undesirable.
+
+Presumably, the runtime should provide a standard set of environment variables and also allow the JSON spec to set additional function-specific environment variables. 
+
+See the reference of environment variables generated by WAGI for details: https://github.com/deislabs/wagi/blob/main/docs/environment_variables.md
+
+## Clocks
+
+`clock_time_get` is implemented but untested. `clock_res_get` is unimplemented.
+
+## File System
+
+SLEdge only supports `fd_read` from stdin and `fd_write` to stderr or stdout.
+
+stdin is populated with the body of an HTTP POST request. stdout and stderr are both written in an interleaved fashion into a buffer and sent back to the client as the response body.
+
+Actual access to the file system is unsupported, and sandboxes are not provided any preopened descriptors.
+
+## Poll
+
+`poll_oneoff` is unsupposed because SLEdge serverless functions are short lived. Sandboxed functions are assumed to make blocking reads/writes to stdin/stdout/stderr, and the serverless runtime is responsible for causing serverless functions to sleep and wake.
+
+## Exit
+
+`proc_exit` is supported and causes a sandbox to terminate execution.
+
+## Signals
+
+`proc_raise` is not supported. Signals are used by the runtime to provide preemption and context switching. It would be dangerous to trigger actual host signals from a sandbox. 
+
+However, the function could be implemented by creating a switch on the wasi signal and either ignoring or handling the signal within the `proc_raise` function itself.
+
+`SIGABRT` could trigger the sandbox to exit in an abnormal condition.
+
+The default ignore behavior could log the unexpected signal and return.
+
+## Random
+
+`random_get` is supported but largely untested.
+
+## Yield
+
+`sched_yield` is unsupported. This does not match with the run-to-completion nature of serverless. 
+
+In the case of EDF, a sandbox would always yield to itself. However, in the case of FIFO, we could enable this call to allow for a worker to "round robin" within a runqueue. However, it is unclear what the rationale would be to allow a serverless function to impact the scheduler.
+
+## Sockets
+
+All socket syscalls are unimplemented because the current logic around `sock_accept` and `sock_shutdown` seems to be focused on long-lived daemon nanoprocesses that handle multiple requests. The `poll_oneoff` call also seems to be based on this usecase. 
+
+Generally, a serverless function is expected to only make outbound network requests. However, this use case does not seem to be currently supported by WASI.