os/exec: edit comment to remove invalid link

Update comment to remove link formatting that doesn't turn into a link, because the target field is not a top-level member of the package. Re-word comment slightly.

Change-Id: I43ebd8fb105b772a4362c0c763e6464321a92747
Reviewed-on: https://go-review.googlesource.com/c/go/+/631856
Reviewed-by: Veronica Silina <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: bitlux

src/os/exec/exec.go

@@ -1000,7 +1000,9 @@ func (c *Cmd) awaitGoroutines(timer *time.Timer) error {
 
 // Output runs the command and returns its standard output.
 // Any returned error will usually be of type [*ExitError].
-// If c.Stderr was nil, Output populates [ExitError.Stderr].
+// If c.Stderr was nil and the returned error is of type
+// [*ExitError], Output populates the Stderr field of the
+// returned error.
 func (c *Cmd) Output() ([]byte, error) {
 	if c.Stdout != nil {
 		return nil, errors.New("exec: Stdout already set")