What are slash commands?

[brianhuster]

Well, I've already used slash commands in agents like Gemini, Copilot, but would still like the ACP documentation to explain it further:

• Must a slash command be in the start of a line?
• Must a prompt contain slash commands only?
• Can a prompt contain many slash commands?

Clarifying this will make it easier for editors/UI to implement completion / signature help for slash commands