docs: amd-memory-encryption.rst get rid of warnings

Get rid of those warnings:

    Documentation/virtual/kvm/amd-memory-encryption.rst:244: WARNING: Citation [white-paper] is not referenced.
    Documentation/virtual/kvm/amd-memory-encryption.rst:246: WARNING: Citation [amd-apm] is not referenced.
    Documentation/virtual/kvm/amd-memory-encryption.rst:247: WARNING: Citation [kvm-forum] is not referenced.

For references that aren't mentioned at the text by adding an
explicit reference to them.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/virtual/kvm/amd-memory-encryption.rst

@@ -241,6 +241,9 @@ Returns: 0 on success, -negative on error
 References
 ==========
 
+
+See [white-paper]_, [api-spec]_, [amd-apm]_ and [kvm-forum]_ for more info.
+
 .. [white-paper] http://amd-dev.wpengine.netdna-cdn.com/wordpress/media/2013/12/AMD_Memory_Encryption_Whitepaper_v7-Public.pdf
 .. [api-spec] http://support.amd.com/TechDocs/55766_SEV-KM_API_Specification.pdf
 .. [amd-apm] http://support.amd.com/TechDocs/24593.pdf (section 15.34)