Fix FileStream.FlushAsync documentation to correctly describe its behavior

[Copilot]

The documentation for FileStream.FlushAsync(CancellationToken) incorrectly stated that it "causes any buffered data to be written to the underlying device". This is inaccurate based on the behavior change implemented in dotnet/coreclr#24902.

Issue

The current documentation was misleading because:

• FlushAsync() behaves like Flush()/Flush(false) - it only flushes .NET stream buffers to the file
• It does not flush intermediate OS file buffers to the underlying storage device
• Only Flush(true) actually performs the expensive operation of flushing OS buffers to disk

Changes

• Updated the summary to remove "underlying device" and clarify it writes "to the file"
• Added explicit documentation in the remarks section explaining that this method only flushes .NET stream buffers
• Added guidance to use Flush(true) when flushing to the underlying storage device is required

This ensures developers have accurate expectations about the performance and behavior of FlushAsync() versus Flush(true).

Fixes #11433.

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.