xz man page: Describe the source file deletion in -z and -d options

xz man page: Describe the source file deletion in -z and -d options

The DESCRIPTION section always explained it, and the OPTIONS section
only described the differences to the default behavior. However, new
users in a hurry may skip reading DESCRIPTION. The default behavior
is a bit dangerous, thus it's good to repeat in --compress and
--decompress docs that source file is removed after successful operation.

Fixes: https://github.com/tukaani-project/xz/issues/150
(cherry picked from commit 653732bd6f06d8f465bf353bf6e1c16f1405b906)