[mdc-theme] Contradicting advice on purpose of `$secondary`

[merchako]

Bug report

You documentation gives conflicting advice on what to use the $secondary color for.

Your readme.md says,

secondary | Used for text which is lower in the visual hierarchy (e.g., text-secondary-on-light)

And your Theming Guide says

The secondary color is used for floating action buttons and other interactive elements, serving as visual contrast to the primary.

The latter suggests that the color is higher (or the same) on the visual hierarchy while getting used less often.

Please clarify.