os: document error returned by pending I/O operations on Close

Currently, File.Close only documents that "an" error will be returned
by pending I/O operations. Update the documentation to say that error
is specifically ErrClosed.

Change-Id: Ica817c9196ad6cb570c826789d37a4ff15a5d13d
Reviewed-on: https://go-review.googlesource.com/c/go/+/371015
Trust: Austin Clements <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: aclements

src/os/file_plan9.go

@@ -139,7 +139,7 @@ func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 
 // Close closes the File, rendering it unusable for I/O.
 // On files that support SetDeadline, any pending I/O operations will
-// be canceled and return immediately with an error.
+// be canceled and return immediately with an ErrClosed error.
 // Close will return an error if it has already been called.
 func (f *File) Close() error {
 	if err := f.checkValid("close"); err != nil {

src/os/file_posix.go

@@ -16,7 +16,7 @@ func sigpipe() // implemented in package runtime
 
 // Close closes the File, rendering it unusable for I/O.
 // On files that support SetDeadline, any pending I/O operations will
-// be canceled and return immediately with an error.
+// be canceled and return immediately with an ErrClosed error.
 // Close will return an error if it has already been called.
 func (f *File) Close() error {
 	if f == nil {