[feat] alter set_allowed_mentions

[folospior]

This allows the developer to use only one parameter if they only require to change the first parameter and leave the others as default. Altered comments.
Example:
dpp::message a = dpp::message().set_allowed_mentions(true); // this allows them to parse users but leave everything else as false

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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[braindigitalis]

Looks good to me, thanks for your first PR :)