Add note for Node Added Trigger documentation in Autoscaling

Node Added Trigger doesn't abide by SOFT rules [strict: false] and results in abnormal cluster operational behavior.

Let's say; we wish to do the following:
1. Not more than 10 cores reside on Single node.
2. Wish to distribute the cores, replicas equally to each Node.

If we go by the following policy:

not more than one replica for unique shard on a node. not a strict rule.

  {"replica":"<2", "shard": "#EACH", "node": "#ANY", "strict": false},

distribute the replicas equally across the nodes, not a strict rule.

  {"replica": "#EQUAL", "node": "#ANY", "strict": false},

not more than 10 cores allowed on a single node, strict rule.

  {"cores": "<10", "node": "#ANY"}

cluster state ends up like:

Screenshot -1

Only the strict rule is followed and multiple replicas are added to single Solr node, as rules are not strict – _

_

While the following with all strict rule generate normal operational behavior, add a replica to each shard of collection 'wiki' :

[
  {"replica":"<2", "shard": "#EACH", "node": "#ANY"},
  {"replica": "#EQUAL", "node": "#ANY"},
  {"cores": "<10", "node": "#ANY"}
  ]

Screenshot -2

This behavior should be documented.