Docs: Document --inspect option in the man page

The `--inspect` option is documented in the osbuild usage, but not in
the man page. This makes it harder for new users to find it and use it
for figuring out stage IDs necessary for checkpoints.

Signed-off-by: Tomas Hozza <thozza@redhat.com>

docs/osbuild.1.rst

@@ -47,6 +47,8 @@ is not listed here, **osbuild** will deny startup and exit with an error.
                                 build (can be passed multiple times)
 --json                          output results in JSON format
 --output-directory=DIR          directory where result objects are stored
+--inspect                       return the manifest in JSON format including
+                                all the ids
 
 NB: If neither ``--output-directory`` nor ``--checkpoint`` is specified, no
 attempt to build the manifest will be made.