[Server] Clarify number of index entries per array

From the docs it wasn't clear to me that the number of index entries for an array field is based on the number of distinct values in the array.

I read the docs below to imply that the number of entries is based on the number of values in the array.

For example, this array [1,1,2] will generate 2 index entries, not 3. This was highlighted in NHTT training and I think the docs should explain it.

https://docs.mongodb.com/manual/core/index-multikey/
"To index a field that holds an array value, MongoDB creates an index key for each element in the array. "

https://docs.mongodb.com/manual/indexes/
"MongoDB creates separate index entries for every element of the array"