QMP: TPM QMP and man page documentation updates

Signed-off-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/qemu-options.hx b/qemu-options.hx
index d7afeab..c40ba55 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -2241,7 +2241,8 @@
 @option{passthrough}.
 
 The specific backend type will determine the applicable options.
-The @code{-tpmdev} option requires a @code{-device} option.
+The @code{-tpmdev} option creates the TPM backend and requires a
+@code{-device} option that specifies the TPM frontend interface model.
 
 Options to each backend are described below.