Mention mathematical special functions

[bernhardmgruber]

Not super important. But people should rather use the standard ones than write them themselves.

See here: https://en.cppreference.com/w/cpp/numeric/special_functions

[stale[bot]]

This issue or pull request has been automatically marked as stale because it has not had recent activity. Please manually close it, if it is no longer relevant, or ask for help or support to help getting it unstuck. Let me bring this to the attention of @klieret @wdconinc @michmx for now.