[docs] Migration guide from v4 to next should mention the color difference of Paper and related components in the dark mode

[ti1024]

In Material-UI next, Paper surface color becomes brighter in the dark mode by default (cf. https://next.material-ui.com/components/paper/#elevation, #18309, and note that the default value of elevation is 1). This also affects other components based on Paper such as Drawer. It is better if these changes are documented in the migration guide (https://next.material-ui.com/guides/migration-v4/).

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

I do not think that https://next.material-ui.com/guides/migration-v4/ mentions the color difference of Paper or Drawer (or any other components that are based on Paper) in the dark mode.

Expected Behavior 🤔

https://next.material-ui.com/guides/migration-v4/ should mention the color difference of Paper and Drawer (and other components that are based on Paper) in the dark mode.

[oliviertassinari]

It was recently added, not released yet: https://next--material-ui.netlify.app/guides/migration-v4/#paper.