Merge bitcoin#926: secp256k1.h: clarify that by default arguments mus…

…t be != NULL

0881633 secp256k1.h: clarify that by default arguments must be != NULL (Jonas Nick)

Pull request description:

  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.

  This line is extracted from bitcoin#783 in the hope that it gets merged faster because other PRs depend on it.

ACKs for top commit:
  gmaxwell:
    ACK 0881633
  real-or-random:
    ACK 0881633

Tree-SHA512: ecdc6954a1c21c333da5b03db51f50a0e53984aaef69cc697adaddc96b276da23e342037f476d21742632f6ec02bfa0574f837a5b5791f5985f4c355037176fa

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.