[Documentation] Clarify reserved special purpose namespaces #908

We need to improve the documentation to clearly state - preferable in one location which namespaces are reserved for special purpose. Those are

Registered namespaces:

• example
• test
• (hopefully soon) invalid (as per Reserve registered namespace invalid #907)

Unregistered namespaces:

• anything starting with x_example., especially x_example.test#test
• (hopefully soon) anything starting with x_invalid.test (as per Reserve registered namespace invalid #907)

This will aid in guiding implementers (and came up during oasis-tcs/csaf#1152 (relates to #802) - but just documentation - so not time critical)).