[Docs] Add note that AGENTS.md is not reloaded after start

### Describe the feature or problem you'd like to solve

Copilot CLI does not reload the AGENTS.md file after it has been initialized, this is not mentioned anywhere.

### Proposed solution

Add a section to the docs https://docs.github.com/en/copilot/how-tos/use-copilot-agents/use-copilot-cli#use-custom-instructions that states that if you edit the `AGENTS.md` file you need to quite and restart copilot cli.

### Example prompts or workflows

_No response_

### Additional context

_No response_