[S24] clarified what TAP doesn't like

It doesn't like no calls to C<plan> or C<done>, it doesn't like more than
one call, it doesn't like a call at an inappropriate time...

S24-testing.pod

@@ -14,7 +14,7 @@ Synopsis 24: Testing
     Created: 30 Dec 2010
 
     Last Modified: 31 Dec 2010
-    Version: 4
+    Version: 5
 
 =head1 SYNOPSIS
 
@@ -52,8 +52,12 @@ number of tests. A TAP harness can then flag an error condition when the number
 of tests actually run doesn't match.
 
 If C<plan> isn't called, C<done> can be called at the end of the test
-file to output an automatically computed tally. A TAP harness will consider
-it an error if neither C<plan> nor C<done> was called.
+file to output an automatically computed tally.
+
+A TAP harness will consider it an error if neither C<plan> nor
+C<done> was called, or if there was more than one call in the test
+file, or if the call occurred between calling two test functions
+(rather than at the beginning or at the end).
 
 =head2 Test functions