Documentation request: expand the SPI acronym when it is introduced

The "SPI" acronym is first introduced in [this section](https://docs.spring.io/spring-batch/reference/scalability.html#partitioning). I don't know what this stands for. AI gives this definition: 

> On that Spring Batch page, SPI stands for Service Provider Interface — specifically, the Partitioner SPI that you implement to split a step execution into partitions.