There is a little documentation on that style RTC module if you search specifically for the Raspberry Pi, because that is what it is designed for. Be careful, the battery may not be a standard coin cell, but instead either a rechargeable battery or a high-capacity capacitor.
I've found a few schematics, with several having obvious errors such as the battery + terminal connecting to one of the I2C pullup resistors (not good when the I2C line goes low), or the two I2C pullups connected ONLY to the battery (presumably to charge the battery), but NOT connected to the I2C lines.