Update Documentation on Labels to reflect the limitation on special characters

Issue Summary

Currently, our documentation does not mention the limitation the label system has when dealing with special characters. This can lead customers to believe that this limitation does not exist. There is a feature request that aims to actually implement the usage of Special Characters in labels:

• CONFCLOUD-6008
• CONFSERVER-6008

This is the documentation that needs to be updated:

• Use Labels to Organize your Content
• Add, Remove and Search for Labels