5680 fix errors in schematron variable substitution support text & example

We discussed this in the conf call on 6/19. We decided to continue the discussion in email.

For reference, we discussed the following on 6/19:

The proposed text says that using sch:let is not sufficient if a message is to be reused outside the context of a schematron constraint. However, the text subsequently uses a schematron based example. Further, the example shows a message that uses sch:value-of in it. Such message cannot be used outside the context of schematron. I think it should use a non-schematron example to be consistent with the earlier text.