man: fix discriptions for --exec-delay

man: fix discriptions for --exec-delay

There are two ambiguity in the original description:
1. It will delay all RUN instructions, include builtin.
2. It will delay before running RUN, not each of RUN{program} instructions.

(cherry picked from commit 45f5efdea7e5e94bd47fc24b9bd404c77b5771a0)
(cherry picked from commit cb92f5601ad169e8f86a61319b73a8fd9e19950b)
(cherry picked from commit 58a56b6a14d5cc9b7ecdd397d7ab6b76170fad08)