Add ability to add Contextual Help to the Theme Customizer

[wpsmith]

While this is probably more complicated than I realize, it would be good to add the ability for contextual help in the admin side of the Theme Customizer. This would enable explanations of the various core features as well as any additional custom features by theme developers.

[ocean90]

You can add a description field to each of your section. There you could include also some explanations.

I don't think the contextual help tab makes sense here.

[obenland]

Related: #24392

[westonruter]

In #27981, customize controls now support descriptions by default as well.