Update docs to include the description of what `legacy` actually does for `@babel/plugin-proposal-decorators`

The docs are very vague about `legacy` flag on `@babel/plugin-proposal-decorators`.
They don't tell which one should a developer choose: `legacy: false` or `legacy: true`.
There is no description of the actual difference in behaviour whether the flag is enabled or disabled.
https://babeljs.io/docs/en/next/babel-plugin-proposal-decorators.html