Add documentation for IRequestExecutorResolver

[zliebersbach]

Is your feature request related to a problem? Please describe.
Frustrated by missing docs on IRequestExecutorResolver. Logging this issue so maybe one day we can have better docs for HotChocolate. See Slack post here: https://hotchocolategraphql.slack.com/archives/CD9TNKT8T/p1625555650042100

Describe the solution you'd like
Some docs so people in the future know about IRequestExecutorResolver.

Describe alternatives you've considered
N/A

Additional context
N/A

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.