[docs] [issue21510] fma documentation should provide better example.

Mark Dickinson added the comment: I wouldn't want to drop the simple example: I suspect that many of those looking at fma won't have the first idea what it does, and that first example shows clearly that it's a fused multiply-add. But +1 for an example that demonstrates the single rounding, either in the online docs or the docstring (or both). ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue21510> _______________________________________