Adds indexing for sinking

Which had already been documented in #732. Closes #1309.

Author: JJ

doc/Language/contexts.pod6

@@ -15,6 +15,7 @@ my $sub = -> $a { return $a² };
 $sub; # OUTPUT: «WARNINGS:␤Useless use of $sub in sink context (line 1)␤»
 =end code
 
+X<|sinking>
 You can force that sink context on L<Iterator>s, by using the L<C<sink-all>|/routine/sink-all> method. L<Proc>s can also be L<sunk via the C<sink> method|https://docs.perl6.org/type/Proc#method_sink>, forcing them to raise an exception and not returning anything.
 
 In general, blocks will warn if evaluated in sink context; however, L<gather/take blocks|/language/control#Flow%29_gather_take> are explicitly evaluated in sink context, with values returned explicitly using C<take>.