secp256k1.h: clarify that by default arguments must be != NULL

The same file says that the illegal callback will only triger for violations
explicitly mentioned, which is not true without this commit because we often
don't mention that an argument is not allowed to be NULL.

include/secp256k1.h

@@ -7,7 +7,9 @@ extern "C" {
 
 #include <stddef.h>
 
-/* These rules specify the order of arguments in API calls:
+/* Unless explicitly stated all pointer arguments must not be NULL.
+ *
+ * The following rules specify the order of arguments in API calls:
  *
  * 1. Context pointers go first, followed by output arguments, combined
  *    output/input arguments, and finally input-only arguments.