sed: add deprecation notice for -i .bak

[emaste]

Summary

PR: 254091

Test Plan

Needs test suite update, posted for discussion.

[tobik]

usr.bin/sed/main.c

142

I think the warning sounds like an error and it is not very clear that sed actually goes ahead and changes the file anyway.

[val_packett.cool]

usr.bin/sed/main.c

138

Hmm. Still considering -i '' non-deprecated — would that really facilitate a transition to the style everyone else uses?