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".