Add "For Implementers" In Documentation For `Log::flush()`

[AldaronLau]

There is almost no information / clarity on what flush() should do and if it's called by any of the macros in this crate. I think it should say something like:

This method isn’t called automatically by the log! macros. It can be called manually at shutdown.

[KodrAus]

Thanks for the suggestion @AldaronLau! This sounds like a nice clarification to me.