SMM: Document immutabilty of Mapping objects

In SMM, Mapping objects are immutable. Any operation that changes the Mappings returns a new Mapping object with updated version id; the old mapping object is now unusable. Some users have found that this design is not very obvious. User education can be improved by doing the following:

a) improving the error message to make it very obvious how to fix the code (e.g. "did you mean to....")

b) on the /// documentation for MarkMappingOffline and MarkMappingOnline noting these methods make existing references invalid, but the return value provides a new reference to the mapping

c) on the /// doc for DeleteMapping recommending use of pattern sm.DeleteMapping(sm.MarkMappingOffline(rm)) if you are deleting right after marking offline

Start by locating the error message and the documentation for MarkMappingOffline, MarkMappingOnline, and DeleteMapping. Update them to explain that marking invalidates existing references, that the return value is the new reference, and that the suggested deletion pattern can be used; done means all three user-education requests are covered.

Written by the indexing model from the issue text.