This is kinda confusing to read, and doesn't explain or demonstrate the purpose of the operator. How about something like:

```
Runs `tryBlock` with each of the receiver's values, propagating any error that
occurs.

tryBlock - An action to run against each value. The block should return whether
           the action succeeded, and set `*error` with any error that occurs.
           This block must not be nil.

Examples

    [signal try:^(NSData *data, NSError **errorPtr) {
        // Causes the returned signal to fail if any data value cannot be
        // written.
        return [data writeToURL:someFileURL options:NSDataWritingAtomic error:errorPtr];
    }];

Returns a signal which passes through the values of the receiver. If `tryBlock`
fails for any value, the returned signal will error using any `NSError` passed
out from the block.
```