[Patch, Fortran] PR 82018: -Wextra should imply -Wconversion-extra

[Thomas Koenig]

Hi Janus,

> here is a small patch that enables -Wconversion-extra with -Wextra and
> updates the documentation.

I grepped for warn_conversion_extra and found 14 occurrences in the
gfortran source tree.

Are we sure we want to enable each of these warnings with -Wextra?

Regards

	Thomas