Guidance for %g equivalent

[jason-s]

C/C++, Python, and Java all support the %g formatting specifier, which effectively selects between %e and %f as appropriate. I was surprised to learn that Rust does not.

If this library is trying to stay close to Rust std::fmt, I wouldn't change the implementation, but please provide some guidance in the documentation for those of us who are used to specifying %g. (either we can't do it, or we should consider a workaround = ???)