add some more docs

Author: dsherret

src/sys.rs

@@ -74,6 +74,10 @@ pub trait Sys: Clone {
     /// Gets and parses the PATHEXT environment variable on Windows.
     ///
     /// Override this to disable globally caching the parsed PATHEXT.
+    /// 
+    /// Note: This will only be called when `is_windows()` returns `true`
+    /// and isn't conditionally compiled with `#[cfg(windows)]` so that it
+    /// can work in Wasm.
     fn env_windows_path_ext(&self) -> Cow<'static, [String]> {
         use std::sync::OnceLock;