Docs: Future-proof comments referencing 5.0 TODOs.

Some Customizer-related files contain inline comments referencing work to do for #42364 for 5.0. Those comments are now outdated as #42364 was moved to the 5.1 milestone.

Merges [43887] to trunk.

Props dlh, pento.
Fixes #45115.
Built from https://develop.svn.wordpress.org/trunk@44228


git-svn-id: https://core.svn.wordpress.org/trunk@44058 1a063a9b-81f0-0310-95a4-ce76da25c4cd