[Str] implement adverbs for .subst

lib/Str.pod

@@ -364,6 +364,25 @@ Here are other examples of usage:
 	
 	$str.subst(/foo/, "bar", :nth(3)); # replace nth match alone. Replaces the third foo. Returns Hey foo foo bar
 
+The following adverbs are supported
+
+=begin table
+
+    short       long        meaning
+    =====       ====        =======
+    :g          :global     tries to match as often as possible
+    :nth(Int)               only substitute the nth's match
+    :ss         :samespace  preserves whitespace on subsitution
+    :ii         :samecase   preserives case on substitution
+    :x(Int)                 substitute exactly $x matches
+
+=end table
+
+Note that only in the C<s///> form C<:ii> implies C<:i> and C<:ss> implies
+C<:s>. In the method form, the C<:s> and C<:i> modifiers must be added to the
+regex, not the C<subst> method call.
+
+
 =head2 substr
 
     multi sub    substr(Str:D $s, Int:D $from, Int:D $chars = $s.chars - $from) returns Str:D