doc: Clarify cert_create build when USE_TBBR_DEFS=0

The user guide documentation for the cert_create tool needs to
mention that a platform must have a platform_oid.h header file
in order to successfully build the cert_create tool when
USE_TBBR_DEFS is 0.

Change-Id: I77f86a022d207e88a79c97741be3eafbfa0c86f1
Signed-off-by: Paul Beesley <paul.beesley@arm.com>

docs/user-guide.rst

@@ -1219,11 +1219,13 @@ command:
     make PLAT=<platform> [DEBUG=1] [V=1] certtool
 
 For platforms that require their own IDs in certificate files, the generic
-'cert_create' tool can be built with the following command:
+'cert_create' tool can be built with the following command. Note that the target
+platform must define its IDs within a ``platform_oid.h`` header file for the
+build to succeed.
 
 ::
 
-    make USE_TBBR_DEFS=0 [DEBUG=1] [V=1] certtool
+    make PLAT=<platform> USE_TBBR_DEFS=0 [DEBUG=1] [V=1] certtool
 
 ``DEBUG=1`` builds the tool in debug mode. ``V=1`` makes the build process more
 verbose. The following command should be used to obtain help about the tool: