rbd docs: document cp versus deep cp

I found that the difference between "rbd cp" and "rbd deep cp",
i.e. what "deep" means in this context, is documented only in
the mailing list archive and in the Mimic reelase notes.

Let's make the difference explicit in the manpage and in rbd --help.

Signed-off-by: Jan "Yenya" Kasprzak <kas@fi.muni.cz>

doc/man/8/rbd.rst

@@ -247,6 +247,7 @@ Commands
 
 :command:`deep cp` (*src-image-spec* | *src-snap-spec*) *dest-image-spec*
   Deep copy the content of a src-image into the newly created dest-image.
+  Deep copy will copy all the image snapshots as well as the "head".
   Dest-image will have the same size, object size, image format, and snapshots as src-image.
 
 :command:`device list` [-t | --device-type *device-type*] [--format plain | json | xml] --pretty-format

src/test/cli/rbd/help.t

@@ -26,7 +26,7 @@
       config pool set                   Set a pool-level configuration override.
       copy (cp)                         Copy src image to dest.
       create                            Create an empty image.
-      deep copy (deep cp)               Deep copy src image to dest.
+      deep copy (deep cp)               Deep copy (with snapshots) src to dest.
       device list (showmapped)          List mapped rbd images.
       device map (map)                  Map an image to a block device.
       device unmap (unmap)              Unmap a rbd device.

src/tools/rbd/action/Copy.cc

@@ -187,7 +187,7 @@ int execute_deep(const po::variables_map &vm,
 }
 
 Shell::Action action_deep(
-  {"deep", "copy"}, {"deep", "cp"}, "Deep copy src image to dest.",
+  {"deep", "copy"}, {"deep", "cp"}, "Deep copy (with snapshots) src to dest.",
   at::get_long_features_help(), &get_arguments_deep, &execute_deep);
 
 } // namespace copy