start section on substitution RHS restrictions

More specifics need to be added.

Author: tbrowder

doc/Language/regexes.pod6

@@ -1610,8 +1610,21 @@ backslash C<\> or included in quotes:
     s/Space\:/Party like it's/;
     .say                        # OUTPUT: «Party like it's 1999␤»
 
-Note that the matching restrictions only apply to the left-hand side of the
-substitution expression.
+Note that the matching restrictions generally only apply to the left-hand side of the
+substitution expression, but some special characters or combinations of them may need
+to be escaped. For example
+
+    $_ = 'foo';
+    s/foo/\%(/;
+    .say        # OUTPUT: <<%(\n>>
+
+or escape the '(' instead for the same result
+
+    s/foo/%\(/;
+    .say        # OUTPUT: <<%(\n>>
+
+but using either character alone does not require escaping. Forward slashes will need
+to be escaped, but escaping alphanumeric characters will cause them to be ignored.
 
 By default, substitutions are only done on the first match: