[PATCH] doc: mention that df -a includes duplicate file systems

[Pádraig Brady]

* 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
---
 doc/coreutils.texi |   10 +++++++---
 src/df.c           |    2 +-
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index aece931..03a4a33 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -11231,9 +11231,13 @@ The program accepts the following options.  Also see 
@ref{Common options}.
 @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.  Inaccessible file systems
+are those which are mounted but subsequently over-mounted by another
+file system at that point.
 
 @item -B @var{size}
 @itemx --block-size=@var{size}
diff --git a/src/df.c b/src/df.c
index ccb1f5e..3c8cc82 100644
--- a/src/df.c
+++ b/src/df.c
@@ -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 dummy and duplicate 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\
-- 
1.7.7.6