Fix documentation for the `-Wunused-packages` flag
There's -Wunused-packages
flag in GHC. Its documentation says that it's available since GHC-8.8.
However, GHC user guide doesn't display this warning. And GHC-8.8 doesn't know about this flag. Does it mean that the documentation is wrong and we should expect to see this flag in GHC-8.10?