[ruby/prism] Provide more documentation for pm_integer_parse_digit_va…

…lues

ruby/prism@c3fcb5031f

prism/util/pm_integer.c

@@ -223,6 +223,10 @@ karatsuba_multiply(pm_integer_t *destination, pm_integer_t *left, pm_integer_t *
 
 /**
  * The values of a hexadecimal digit, where the index is the ASCII character.
+ * Note that there's an odd exception here where _ is mapped to 0. This is
+ * because it's possible for us to end up trying to parse a number that has
+ * already had an error attached to it, and we want to provide _something_ to
+ * the user.
  */
 static const int8_t pm_integer_parse_digit_values[256] = {
 //   0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F