doc: df, du, ls: improve --blocksize description in --help

* src/du.c (usage): Print better --blocksize description.
Prompted by Samuel Thibault in <http://bugs.debian.org/353100>.
* src/df.c (usage): Likewise.
* src/ls.c (usage): Likewise.

src/df.c

@@ -819,7 +819,9 @@ Mandatory arguments to long options are mandatory for short options too.\n\
 "), stdout);
       fputs (_("\
   -a, --all             include dummy file systems\n\
-  -B, --block-size=SIZE  use SIZE-byte blocks\n\
+  -B, --block-size=SIZE  scale sizes by SIZE before printing them.  E.g.,\n\
+                           `-BM' prints sizes in units of 1,048,576 bytes.\n\
+                           See SIZE format below.\n\
       --total           produce a grand total\n\
   -h, --human-readable  print sizes in human readable format (e.g., 1K 234M 2G)\n\
   -H, --si              likewise, but use powers of 1000 not 1024\n\

src/du.c

@@ -281,7 +281,9 @@ Mandatory arguments to long options are mandatory for short options too.\n\
                           fragmentation, indirect blocks, and the like\n\
 "), stdout);
       fputs (_("\
-  -B, --block-size=SIZE  use SIZE-byte blocks\n\
+  -B, --block-size=SIZE  scale sizes by SIZE before printing them.  E.g.,\n\
+                           `-BM' prints sizes in units of 1,048,576 bytes.\n\
+                           See SIZE format below.\n\
   -b, --bytes           equivalent to `--apparent-size --block-size=1'\n\
   -c, --total           produce a grand total\n\
   -D, --dereference-args  dereference only symlinks that are listed on the\n\

src/ls.c

@@ -4592,7 +4592,9 @@ Mandatory arguments to long options are mandatory for short options too.\n\
   -b, --escape               print C-style escapes for nongraphic characters\n\
 "), stdout);
       fputs (_("\
-      --block-size=SIZE      use SIZE-byte blocks.  See SIZE format below\n\
+      --block-size=SIZE      scale sizes by SIZE before printing them.  E.g.,\n\
+                               `--block-size=M' prints sizes in units of\n\
+                               1,048,576 bytes.  See SIZE format below.\n\
   -B, --ignore-backups       do not list implied entries ending with ~\n\
   -c                         with -lt: sort by, and show, ctime (time of last\n\
                                modification of file status information)\n\