[Overlay] Clarify style/theme resolution/throwing in the documentation

[Westbrook]

Things projecting into an active-overlay element expect to benefit from either a global application of Spectrum CSS custom properties or have lived within an sp-theme scope in order to have their theme context resolve appropriately while the overlay is open, clarify this in the docs.

Styles applied to content thrown into an overlay will not be associated with that content when an overlay is open. Clarify the requirements of an encapsulated component, inline styles, or a <style> element contained in the thrown content.

cc: @aashritandon92

[Westbrook]

@aashritandon92 I've added the proposed Styling section to https://westbrook-overlay-styles--spectrum-web-components.netlify.app/components/overlay-trigger does it seem clearly outline the options we discussed oh so long ago now?

[aashritandon92]

@aashritandon92 I've added the proposed Styling section to https://westbrook-overlay-styles--spectrum-web-components.netlify.app/components/overlay-trigger does it seem clearly outline the options we discussed oh so long ago now?

It looks good to me. Thanks for adding that!