random: mark RandAddPeriodic and SeedPeriodic as noexcept
Summary:
The usage of MilliSleep() in SeedPeriodic (previously SeedSleep) was removed in #17270, meaning it, and its users can now be marked noexcept. This also corrects the docs in random.h for some of the changes in #17270.
Backport of core PR17507.
Depends on D6209.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D6211