Plugins: Simplify the logic of `validate_plugin_requirements()`, update documentation.

This updates the function for consistency with `validate_theme_requirements()`.

Follow-up to [44978], [45546], [47573].

Fixes #43992.
Built from https://develop.svn.wordpress.org/trunk@47574


git-svn-id: https://core.svn.wordpress.org/trunk@47349 1a063a9b-81f0-0310-95a4-ce76da25c4cd