justify adjust_abi behavior with ms docs

Author: beepster4096

compiler/rustc_target/src/spec/mod.rs

@@ -2402,6 +2402,10 @@ impl Target {
     pub fn adjust_abi(&self, abi: Abi, c_variadic: bool) -> Abi {
         match abi {
             Abi::C { .. } => self.default_adjusted_cabi.unwrap_or(abi),
+
+            // On Windows, `extern "system"` behaves like msvc's `__stdcall`
+            // `__stdcall` only applies on x86 and on non-variadic functions
+            // see https://learn.microsoft.com/en-us/cpp/cpp/stdcall?view=msvc-170
             Abi::System { unwind } if self.is_like_windows && self.arch == "x86" && !c_variadic => {
                 Abi::Stdcall { unwind }
             }