emphasize probs with when

please correct as necessary

doc/Language/control.pod6

@@ -84,6 +84,26 @@ You have to watch out for this in most languages anyway to prevent things
 from getting accidentally commented out.  Many of the examples below may
 have unnecessary semicolons for clarity.
 
+=head1 X<while|control flow>
+
+The C<while> block is not usually a stand-alone block since it most commonly used
+inside C<given>, C<for>, and C<loop> blocks and special details of its usage in
+those contexts are discussed there. However, there are a few subleties in proper
+syntax of the contents inside the block's curly braces worth high-lighting.
+Treat the contents pretty much as any block except for function calls.  (Details are found
+L<here|/language/syntax#Signature_literals>A). For example, calling a help function (C<sub help>):
+
+=begin code
+for @*ARGS {
+    when /:i ^ h/ { &help   }  # error
+    when /:i ^ h/ { &help;  }  # error
+    when /:i ^ h/ { &help() }  # calls sub help
+    when /:i ^ h/ { help    }  # calls sub help
+    when /:i ^ h/ { help()  }  # calls sub help
+    when /:i ^ h/ { help(); }  # calls sub help
+}
+=end code
+
 =head1 X<do|control flow>
 
 The simplest way to run a block where it cannot be a stand-alone statement