Improve documentation around number of FTS partitions

We are about to change the default number of partitions to 1.  As a result of this, we will need to improve the documentation around how to correctly choose the number of partitions.

Some rough guidelines:

• Increasing the number of partitions greater than one can allow multiple nodes running the search service to share in indexing/searching workload.
• This can reduce index build time, and increase search throughput, but may also increase search latency.
• Typically it does not make sense to set the number of partitions greater than the number of nodes running the FTS service.
• However, you cannot change the number of partitions without rebuilding the index.
• Because of this in some cases, it may be appropriate to over-partition to allow taking advantage of future nodes.
• Generally we recommend using the fewest number of partitions need to meet  your applications search performance requirements.