doc: modernize usage of “disk” and “core”

doc: modernize usage of “disk” and “core”

In documentation and comments, don’t assume that secondary storage
devices are disk devices.  Similarly, don’t assume that main memory
uses magnetic cores, which became obsolete in the 1970s.
* src/du.c (usage):
* src/ls.c (usage):
* src/shred.c (usage): Reword to avoid “disk” in usage messages.