[S09] Add the term "semilist" to S09

This way someone seeing the mention of semilist in S03 can search
through the spec and find the place where it's discussed in detail, S09,
just by searching for the term itself, rather than thinking to search
for "semicolon list" or "semicolon-separated list" instead.

S09-data.pod

@@ -389,8 +389,8 @@ supplied entirely by an explicit constructor at run-time:
     my num @nums = Array of num.new(:shape(3;3;3));
     my num @nums .=new():shape(3;3;3); # same thing
 
-A multidimensional array is indexed by a semicolon list, which is really
-a list of lists in disguise. Each sublist is a slice of one
+A multidimensional array is indexed by a semicolon list (or "semilist"), which
+is really a list of lists in disguise. Each sublist is a slice of one
 particular dimension. So:
 
     @array[0..10; 42; @x]