[Stopwatch] Document `__root__` section

[ThomasLandauer]

See symfony/symfony#37630 (comment)

[carsonbot]

Thank you for this issue.
There has not been a lot of activity here for a while. Has this been resolved?

[carsonbot]

Just a quick reminder to make a comment on this. If I don't hear anything I'll close this.

[carsonbot]

Hey,

I didn't hear anything so I'm going to close it. Feel free to comment if this is still relevant, I can always reopen!