docs: document why we filter integer keys from the $headers array

Signed-off-by: Matthew Weier O'Phinney <matthew@weierophinney.net>

Author: weierophinney

src/functions/marshal_headers_from_sapi.php

@@ -66,6 +66,9 @@ function marshalHeadersFromSapi(array $server): array
         }
     }
 
-    // Filter out integer keys
+    // Filter out integer keys.
+    // These can occor if the translated header name is a string integer.
+    // PHP will cast those to integers when assigned to an array.
+    // This filters them out.
     return array_filter($headers, fn(string|int $key): bool => is_string($key), ARRAY_FILTER_USE_KEY);
 }