docs: Improve callback docs

Author: real-or-random

include/secp256k1.h

@@ -350,7 +350,10 @@ SECP256K1_API void secp256k1_context_set_illegal_callback(
 ) SECP256K1_ARG_NONNULL(1);
 
 /** Set a callback function to be called when an internal consistency check
- *  fails. The default is crashing.
+ *  fails.
+ *
+ *  The default callback writes an error message to stderr and calls abort
+ *  to abort the program.
  *
  *  This can only trigger in case of a hardware failure, miscompilation,
  *  memory corruption, serious bug in the library, or other error would can