Document :st, :nd, :rd, and :th adverbs on .subst

doc/Type/Str.pod6

@@ -691,14 +691,21 @@ 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 C<:nth> adverb has readable English-looking variants:
+
+    say 'ooooo'.subst: 'o', 'x', :1st; xoooo
+    say 'ooooo'.subst: 'o', 'x', :2nd; oxooo
+    say 'ooooo'.subst: 'o', 'x', :3rd; ooxoo
+    say 'ooooo'.subst: 'o', 'x', :4th; oooxo
+
 The following adverbs are supported
 
 =begin table
 
     short              long        meaning
     =====              ====        =======
     :g                 :global     tries to match as often as possible
-    :nth(Int|Callable)             only substitute the nth's match
+    :nth(Int|Callable)             only substitute the nth's match; aliases: :st, :nd, :rd, and :th
     :ss                :samespace  preserves whitespace on substitution
     :ii                :samecase   preserves case on substitution
     :mm                :samemark   preserves character marks (e.g. 'ü' replaced with 'o' will result in 'ö')