doc: mention that df -a includes duplicate file systems

* src/df.c (usage): Mention that duplicate file systems are shown
with this option, not just dummy file systems.
* doc/coreutils.texi (df invocation): For the --all option, expand
on the class of normally suppressed mount entries that it includes.

Reported in http://bugs.debian.org/737399

2 files changed, 10 insertions, 6 deletions

diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index aece931f2..f089e435c 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -11229,11 +11229,15 @@ The program accepts the following options.  Also see @ref{Common options}.
 @itemx --all
 @opindex -a
 @opindex --all
-@cindex automounter file systems
 @cindex ignore file systems
-Include in the listing dummy file systems, which
-are omitted by default.  Such file systems are typically special-purpose
-pseudo-file-systems, such as automounter entries.
+Include in the listing dummy, duplicate, or inaccessible file systems, which
+are omitted by default.  Dummy file systems are typically special purpose
+pseudo file systems such as @samp{/proc}, with no associated storage.
+Duplicate file systems are local or remote file systems that are mounted
+at separate locations in the local file hierarchy, or bind mounted locations.
+Inaccessible file systems are those which are mounted but subsequently
+over-mounted by another file system at that point, or otherwise inaccessible
+due to permissions of the mount point etc.
 
 @item -B @var{size}
 @itemx --block-size=@var{size}
diff --git a/src/df.c b/src/df.c
index ccb1f5eb6..5231676da 100644
--- a/src/df.c
+++ b/src/df.c
@@ -648,7 +648,7 @@ filter_mount_list (bool devices_only)
                       > strlen (me->me_mountdir))
                   /* let an entry overmounted on a different device win...  */
                   || (! STREQ (devlist->me->me_devname, me->me_devname)
-                      /* ... but only when matching an exsiting mount point, to
+                      /* ... but only when matching an existing mount point, to
                       avoid problematic replacement when given inaccurate mount
                       lists, seen with some chroot environments for example.  */
                       && STREQ (me->me_mountdir, devlist->me->me_mountdir)))
@@ -1396,7 +1396,7 @@ or all file systems by default.\n\
       /* TRANSLATORS: The thousands and decimal separators are best
          adjusted to an appropriate default for your locale.  */
       fputs (_("\
-  -a, --all             include dummy file systems\n\
+  -a, --all             include pseudo, duplicate, inaccessible file systems\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\