Add output documentation on monte_carlo_integral

[davidlowryduda]

This small commit adds an OUTPUT description to the documentation of monte_carlo_integral. The wording of the output is essentially taken from numerical_integral.

This is a nearly trivial documentation change, made after having a conversation with someone using monte-carlo-integration and being confused at the output. Specifically, the output is the tuple (approximate_integral, approximate_error_bound). The error bound wasn't documented.

I will also note that this is my first direct contribution in the github era - please let me know if there is something I should be doing here.

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.