replace Add example with something more evocative of addition

Currently most of the operator traits use trivial implementation
examples that only perform side effects. Honestly, that might not be too
bad for the sake of documentation; but anyway, here's a proposal to move
a slightly modified version of the module-level point-addition example
into the `Add` documentation, since it's more evocative of addition
semantics.

Part of #29365

wrap identifiers in backticks

minor rephrasing

fix module-level documentation to be more truthful

This branch changes the example for `Add` to no longer be a "minimum implementation that prints something to the screen".

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