Document limitations of XSLT Code Coverage

[galtm]

In https://saxonica.plan.io/issues/6302, Michael Kay mentions some situations where it'll be hard for Saxon to produce a well-formed trace (try/catch and multi-threading). To the extent that those situations also cause XSpec XSLT Code Coverage not to work correctly, we should document them as limitations.

The wiki page about code coverage already has a section about limitations.
https://github.com/xspec/xspec/wiki/Code-Coverage#limitations