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.

tests/rm/ext3-perf.sh

@@ -21,7 +21,7 @@ print_ver_ rm
 
 very_expensive_
 
-# Using rm -rf to remove a 400k-entry directory takes:
+# In a circa 2008 benchmark, using rm -rf to remove a 400k-entry directory took:
 # - 9 seconds with the patch, on a 2-yr-old system
 # - 350 seconds without the patch, on a high-end system (disk 20-30% faster)
 threshold_seconds=60
@@ -32,9 +32,9 @@ n=400000
 # Choose a value that is large enough to ensure an accidentally
 # regressed rm would require much longer than $threshold_seconds to remove
 # the directory.  With n=400k, pre-patch GNU rm would require about 350
-# seconds even on a fast disk.  On a relatively modern system, the
+# seconds even on a fast disk.  On a circa 2006 system, the
 # patched version of rm requires about 10 seconds, so even if you
-# choose to enable very expensive tests with a disk that is much slower,
+# choose to enable very expensive tests with a device that is much slower,
 # the test should still succeed.
 
 # Skip unless "." is on an ext[34] file system.