io/ioutil: update WriteFile to clarify it does not change permissions if the file exists.

The existing documentation of WriteFile does not make it clear for
non-native English speakers that it will not change the permissions if
the file already exists before.

Fixes #35711

Change-Id: If861c3e3700957fc9ac3d5313351c57d399d3f58
Reviewed-on: https://go-review.googlesource.com/c/go/+/218417
Reviewed-by: Rob Pike <[email protected]>

Author: joshuabezaleel

src/io/ioutil/ioutil.go

@@ -75,7 +75,7 @@ func ReadFile(filename string) ([]byte, error) {
 
 // WriteFile writes data to a file named by filename.
 // If the file does not exist, WriteFile creates it with permissions perm
-// (before umask); otherwise WriteFile truncates it before writing.
+// (before umask); otherwise WriteFile truncates it before writing, without changing permissions.
 func WriteFile(filename string, data []byte, perm os.FileMode) error {
 	f, err := os.OpenFile(filename, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
 	if err != nil {