Docs: Improve documentation for `deactivate_plugins()` and `activate_plugins()`.

Props marekdedic, spenserhale, SergeyBiryukov.
Fixes #48352.
Built from https://develop.svn.wordpress.org/trunk@46694


git-svn-id: https://core.svn.wordpress.org/trunk@46494 1a063a9b-81f0-0310-95a4-ce76da25c4cd