Document removing the embedded backtracer

As suggested in https://github.com/google/bloaty/issues/110#issuecomment-2571410292 and discussed in a Reddit post, disabling the backtrace feature in std can reduce the binary size by a fixed amount of ~300 kB. This may be significant for programs aiming to be absolutely minimal, e.g. a "Hello, world!" program can be reduced in size by 83%.

This information could be added to the Abort on Panic section, as the embedded backtracer should be unused when aborting on panic!()s.