Document $*INIT-INSTANT

Fixes #510 Fixes #1462

Rakudo impl: rakudo/rakudo@6bdb2dd368
Spec:        Raku/roast@84b48ec92b

Author: zoffixznet

doc/Language/5to6-perlvar.pod6

@@ -236,9 +236,9 @@ translation.
 
 =item $^T
 
-Replaced in Perl 6 by C<$*INITTIME>. Unlike in Perl 5, this is not in seconds
-since epoch, but an C<Instant> object, which is measured in atomic seconds,
-with fractions.
+Replaced in Perl 6 by C<$*INIT-INSTANT>. Unlike in Perl 5, this is not in
+seconds since epoch, but an L<Instant> object, which is measured in atomic
+seconds, with fractions.
 
 =item $PERL_VERSION
 

doc/Language/variables.pod6

@@ -1099,6 +1099,12 @@ X<|$*SPEC>X<|$*TMPDIR>X<|$*THREAD>X<|$*SCHEDULER>X<|$*USAGE>
     ------------------+--------------------------------------------
     $*REPO            | A variable holding information about modules installed/loaded
     ------------------+--------------------------------------------
+    $*INIT-INSTANT    | An Instant object representing program statup time.
+                      | In particular, this is when the core code starts up,
+                      | so the value of $*INIT-INSTANT may be a few miliseconds
+                      | earlier than C<INIT now> or even C<BEGIN now> executed
+                      | in your program.
+    ------------------+--------------------------------------------
     $*TZ              | The system's local timezone offset, as the number of B<seconds> from GMT
     ------------------+--------------------------------------------
     $*CWD             | The Current Working Directory.