docs/devel/loads-stores.rst: Document our various load and store APIs

QEMU has a wide selection of different functions for doing
loads and stores; provide some overview documentation of
what they do and how to pick which one to use.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1507813181-11860-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>