I don't have a circle-with-arrow-leaving-towards-10-o-clock key :D

That is standard Mac ESC key icon: http://www.danrodney.com/mac/.

Where can one find a reference that shows the mapping of InteliJ (JetBrains) key icons to textual name or key labels. I frequently see key icons in Tip of the Day windows that do not match any key on the keyboard I happen to be using at the time. The ESC key icon baffled me as well. Is there a "describe key" tool in the IntelliJ IDEs? which shows the standard JetBrains icon for that key along with some other text, possibly some aliases?

As it's not specific only to IntelliJ IDEA, you can find the reference in the macOS documentation: https://support.apple.com/guide/mac-help/what-are-those-symbols-shown-in-menus-cpmh0011/mac .

Interesting.  Does this mean that IntelliJ et.al. are platform dependent such that they use key icons that are, hopefully, described in a platform document? Is this information described in a JetBrains document that I probably should have read more closely?

Other platforms do not use key icons in the menus, it's macOS specific.