doc: Remove redundant boot sequence in design document

1- The boot sequence for the RSS is being duplicated.
   Remove the initial description and leave the one under the
   boot process diagram only.
2- There is a sentence which starts with text in parenthesis.
   Move it to the end of the sentence.

Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>