Twenty Fifteen: add help text explaining header/sidebar controls in the Customizer

[celloexpressions]

It isn't immediately obvious why several of Twenty Fifteen's options are for both the header and the sidebar, especially when users will only ever see one at a time. Let's add a description to all of the relevant controls here, explaining that it's a sidebar on larger screens and a header on smaller ones.

[iamtakashi]

Add descriptions to header/sidebar controls in the Customizer

[iandstewart]

More verbose description of responsive design changes and an expanded "and" while #29572 is being worked on

[celloexpressions]

I think we should say that it's a header on smaller screens and a sidebar on larger ones. "May only be applied to the sidebar on wider screens" isn't really accurate because it just applies to the header on smaller ones. "Only applied to the sidebar on wider screens. On small screens the sidebar will become the header" could probably be shortened to just the second sentence.

We also need to add something to the core header image section or control (since it's also a sidebar background image).

[iamtakashi]

Also remove the ampersands.

[iamtakashi]

Additional description to header image section.

[iandstewart]

In 30272:

Twenty Fifteen: adding help text to explain how color changes to header and sidebar are affected by responsive design.

Props iamtakashi, iandstewart, fixes #30165.