Add some explanatory comments to ErrnoGuard

src/catch2/internal/catch_errno_guard.hpp

@@ -9,8 +9,12 @@
 
 namespace Catch {
 
+    //! Simple RAII class that stores the value of `errno`
+    //! at construction and restores it at destruction.
     class ErrnoGuard {
     public:
+        // Keep these outlined to avoid dragging in macros from <cerrno>
+
         ErrnoGuard();
         ~ErrnoGuard();
     private: