docs/devel/memory.txt: Document _with_attrs accessors

When we added the _with_attrs accessors we forgot to mention
them in the documentation.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20180824170422.5783-4-peter.maydell@linaro.org>
Based-on: <20180802174042.29234-1-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>