Fix docs for AS::Notifications::Event#duration

Stems from comments by @zzak on e07f3dd:

* https://github.com/rails/docrails/commit/e07f3ddcac394d2a8dc23fc571318b7e8c2497b1#commitcomment-9015634
* https://github.com/rails/docrails/commit/e07f3ddcac394d2a8dc23fc571318b7e8c2497b1#commitcomment-9015639

[ci skip]
Add docs for Time#find_zone

Also improves docs for `Time#find_zone!`

[ci skip]
Add docs for Numeric#*_bytes methods

Add docs for `kilobytes`, `megabytes`, `gigabytes`, `terabytes`,
`petabytes` and `exabytes`. Fix docs for `bytes`.

[ci skip]