Describe the bug

For Upgrader.Upgrade() the concurrency section of the docs doesn't describe the concurrency characteristics
of this method.

Forgive me if I missed something, I triple read the section on concurrency to see if I could determine the thread safetyness of Upgrader.Upgrade()
But I didn't see anything covering this.

Versions

Go version: N/A
package version: c3dd95a