[v6.d REVIEW] Document how we came up with 1+0i for power

http://colabti.org/irclogger/irclogger_log/perl6?date=2018-07-28#l579

S32-num/complex.t

@@ -258,6 +258,11 @@ subtest 'exponentiation with zero (n = 0+0i)' => {
     plan 3;
 
     # Per discussion on https://rt.perl.org/Public/Bug/Display.html?id=128785
+    # + majority of languages doing the 1+0i and not a NaN
+    #    http://rosettacode.org/wiki/Zero_to_the_zero_power
+    # + IEEE Std 1076.2-1996 IEEE Standard VHDL Mathematical Packages
+    #    https://perso.telecom-paristech.fr/guilley/ENS/20171205/TP/tp_syn/doc/IEEE_VHDL_1076.2-1996.pdf
+    #    that defines EXP operation for complex and does use 1+0i for the zero case
     constant n = 0+0i;
     is-deeply n**0, 1+0i, 'n**0';
     is-deeply n**n, 1+0i, 'n**n';