[Identity] Update Service Fabric testing documentation

[laiapat]

Update managed identity testing documentation for Service Fabric with the following:

• ACR accounts with uppercase letters in their name may have authentication issues -- make a brief warning about this
• Add clearer instructions about how to set up the browser certificate necessary to access the SF Explorer (and specify that it's the certificate created earlier in the sample)
• Add a troubleshooting section that at least covers how to respond to crashing applications/nodes

[xiangyan99]

#21370