clarify `cluster.routing.allocation.disk.watermark.low` docs

[jpcarey]

The docs for the low watermark read as if no shard would be allocated to a node above this threshold. It can still allocate a shard if it is a primary shard AND has never been allocated AND the node is below the high threshold. cc: @jasontedor

cluster.routing.allocation.disk.watermark.low
Controls the low watermark for disk usage. It defaults to 85%, meaning ES will not allocate new shards to nodes once they have more than 85% disk used. It can also be set to an absolute byte value (like 500mb) to prevent ES from allocating shards if less than the configured amount of space is available.
https://www.elastic.co/guide/en/elasticsearch/reference/5.4/disk-allocator.html#disk-allocator

[elasticmachine]

Pinging @elastic/es-distributed