GlobalPlatform ADF.SD: Add command line reference + error message

The get_data shell command didn't have any interactive help / syntax,
and no meaningful error message in case an unknown data object name
was specified by the user.  Let's fix that.

Change-Id: I09faaf5d45118635cf832c8c513033aede1427e5
diff --git a/docs/shell.rst b/docs/shell.rst
index ef08fcf..73b988a 100644
--- a/docs/shell.rst
+++ b/docs/shell.rst
@@ -795,8 +795,9 @@
 
 get_data
 ~~~~~~~~
-Performs the GET DATA command as specified by GlobalPlatform.
-
+.. argparse::
+   :module: pySim.global_platform
+   :func: ADF_SD.AddlShellCommands.get_data_parser
 
 cmd2 settable parameters
 ------------------------