Revert "Docs: Clean for next release"

Revert "Docs: Clean for next release"

Exim 4.89 is a stabilization release with very few new features; 4.88
had enough issues that some folks held off upgrading.  It's more useful
for the changebars in the 4.89 release to be relative to the 4.87
release than 4.88.

This reverts most of commit fd047340363431f15d2b0ac686b9dba4fa125781.