Add comments clarifying behavior of unix set_readonly behavior.

src/libstd/sys/unix/fs.rs

@@ -170,11 +170,17 @@ impl AsInner<stat64> for FileAttr {
 }
 
 impl FilePermissions {
-    pub fn readonly(&self) -> bool { self.mode & 0o222 == 0 }
+    pub fn readonly(&self) -> bool {
+        // check if any class (owner, group, others) has write permission
+        self.mode & 0o222 == 0
+    }
+
     pub fn set_readonly(&mut self, readonly: bool) {
         if readonly {
+            // remove write permission for all classes; equivalent to `chmod a-w <file>`
             self.mode &= !0o222;
         } else {
+            // add write permission for all classes; equivalent to `chmod a+w <file>`
             self.mode |= 0o222;
         }
     }