Document some proposals are not fully implemented

Be sure to warn users that the entirety of some proposals are not
implemented, so bugs should be expected.

Closes #1601

crates/api/src/runtime.rs

@@ -145,6 +145,11 @@ impl Config {
  ///
  /// This is `false` by default.
  ///
+ /// > **Note**: Wasmtime does not implement everything for the wasm threads
+ /// > spec at this time, so bugs, panics, and possibly segfaults should be
+ /// > expected. This should not be enabled in a production setting right
+ /// > now.
+ ///
  /// [threads]: https://github.com/webassembly/threads
  pub fn wasm_threads(&mut self, enable: bool) -> &mut Self {
  self.validating_config.operator_config.enable_threads = enable;
@@ -170,6 +175,11 @@ impl Config {
  ///
  /// This is `false` by default.
  ///
+ /// > **Note**: Wasmtime does not implement everything for the reference
+ /// > types proposal spec at this time, so bugs, panics, and possibly
+ /// > segfaults should be expected. This should not be enabled in a
+ /// > production setting right now.
+ ///
  /// [proposal]: https://github.com/webassembly/reference-types
  pub fn wasm_reference_types(&mut self, enable: bool) -> &mut Self {
  self.validating_config
@@ -196,6 +206,11 @@ impl Config {
  ///
  /// This is `false` by default.
  ///
+ /// > **Note**: Wasmtime does not implement everything for the wasm simd
+ /// > spec at this time, so bugs, panics, and possibly segfaults should be
+ /// > expected. This should not be enabled in a production setting right
+ /// > now.
+ ///
  /// [proposal]: https://github.com/webassembly/simd
  pub fn wasm_simd(&mut self, enable: bool) -> &mut Self {
  self.validating_config.operator_config.enable_simd = enable;