No code changes, only revised comments in an attempt to make
escape analysis slightly less confusing.

Updates #23109.

Change-Id: I5ee6cea0946ced63f6210ac4484a088bcdd862fb
Reviewed-on: https://go-review.googlesource.com/121001
Run-TryBot: David Chase <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Cherry Zhang <[email protected]>