Add a comment to explain how errno is set

Since it is nontrivial which modifies `errno` is modified in
ExecuteForTask(), add a comment that explains this.

Bug: 213617178
Test: Compile-tested only.
Change-Id: I49ce9c8054fdc59e61b2e5f9ffe6f16743a94401
Signed-off-by: Bart Van Assche <bvanassche@google.com>

libprocessgroup/task_profiles.cpp

@@ -215,6 +215,9 @@ bool SetAttributeAction::ExecuteForTask(int tid) const {
                 return false;
             }
         }
+        // The PLOG() statement below uses the error code stored in `errno` by
+        // WriteStringToFile() because access() only overwrites `errno` if it fails
+        // and because this code is only reached if the access() function returns 0.
         PLOG(ERROR) << "Failed to write '" << value_ << "' to " << path;
         return false;
     }