CxlCorErrorType member @retry-threshold is undocumented

[Markus Armbruster]

Hi Jonathan,

You added the type in commit 415442a1b4a (hw/mem/cxl_type3: Add CXL RAS
Error Injection Support.)  The doc comment is missing a description of
@retry-threshold.  Can you supply me one?  I'm happy to do the actual
patch.