doc: uefi: Update the capsule update related documentation

Update the capsule update functionality related documentation to
refect the additional definitions that need to be made per platform
for supporting the capsule update feature.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>