Add documentation about why some votes or Agent actions show "unknown function"

[sohkai]

• Explain why this happens
• Explain how the developer might be able to fix it (e.g. registering on 4bytes.directory, adding it to the hardcoded list in radspec)

[john-light]

@sohkai are you planning to add this documentation yourself?

[sohkai]

@pythonpete32 will be writing and submitting it here, and I can review.