[Bug-cpio] --no-absolute-filenames: remind user to modify pattern

[積丹尼 Dan Jacobson]

In (info "(cpio) Options") we see
  `--no-absolute-filenames'
     Create all files relative to the current directory in copy-in
     mode, even if they have an absolute file name in the archive.

Also mention:
     When using this switch remember to remove any leading / from
     'pattern' too for it to still match.