.NET: Clarify IResettableExecutor usage comment in workflow sample

[peibekwe]

Description

Clarify IResettableExecutor usage comment in workflow sample. There's also a follow-up docs PR to document the interface and recommended usage.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

Updates the .NET workflow sample documentation to better explain when and why IResettableExecutor is needed when reusing workflow instances across multiple runs.

Changes:

• Expanded XML docs in WorkflowFactory.cs to describe shared executor instances and IResettableExecutor expectations.
• Expanded sample-level docs and inline loop comment in Program.cs to explain per-iteration workflow runs and executor reset behavior.
• Minor grammar fix in the sample description (“concept of workflows as agents”).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
dotnet/samples/03-workflows/Agents/WorkflowAsAnAgent/WorkflowFactory.cs	Adds/clarifies XML documentation around shared executors and resetting state between runs.
dotnet/samples/03-workflows/Agents/WorkflowAsAnAgent/Program.cs	Adds sample-level explanation of IResettableExecutor behavior in the interactive loop + minor grammar fix.