[Documentation] Add troubleshooting information to Token cache serialization docs

[gladjohn]

Documentation related to component Token cache serialization in MSAL.NET

• [ ] typo
• [ ] documentation doesn't exist
• [x] documentation needs clarification
• [ ] error(s) in example
• [x] needs example

Description of the issue update the docs, by adding a troubleshooting section

L2 cache operations fail silently.

 options.OnL2CacheFailure = (ex) =>
                            {
                                Trace.WriteLine(ex);
                                return true;
                            };

Internal issue link