Add some more comments around verification fields

Author: sipa

src/field.h

@@ -35,7 +35,10 @@
  * In VERIFY mode, they are materialzed explicitly as fields in the struct,
  * allowing run-time verification of these properties. In that case, the
  * field implementation also provides a secp256k1_fe_check routine to
- * verify that these fields match the run-time value. */
+ * verify that these fields match the run-time value.
+ *
+ * See the code at the bottom of field_impl.h for the exact rules about
+ * propagation of these fields, and checking of them. */
 #ifdef VERIFY
 #  define SECP256K1_FE_VERIFY_FIELDS \
     int magnitude; \

src/field_impl.h

@@ -136,6 +136,8 @@ static int secp256k1_fe_sqrt(secp256k1_fe *r, const secp256k1_fe *a) {
     return secp256k1_fe_equal(&t1, a);
 }
 
+/**************** PROPAGATION/CHECKING OF VERIFICATION FIELDS ************/
+
 static void secp256k1_fe_impl_normalize(secp256k1_fe *r);
 SECP256K1_INLINE static void secp256k1_fe_normalize(secp256k1_fe *r) {
     secp256k1_fe_impl_normalize(r);