Add clarifications to CCA docs

[nhu-do]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

1. https://docs.github.com/en/copilot/how-tos/use-copilot-agents/coding-agent/troubleshoot-coding-agent
2. https://docs.github.com/en/copilot/how-tos/use-copilot-agents/coding-agent/customize-the-agent-environment

What part(s) of the article would you like to see updated?

Minor clarifications on limitations.

Example pieces of feedback these changes will address:

• https://github.com/github/sweagentd/issues/3949
• Can the agent read images? copilot-coding-agent/user-feedback#66

Additional information

No response

[welcome]

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[nhu-do]

@docs team, happy to take this!