Add clarification of which operators allowed in the subdoc api for arrays

In this section there is an example of insert,

https://docs.couchbase.com/java-sdk/current/howtos/subdocument-operations.html#array-insertion

we should add a clarification which operators allowed here, or add a section,
for example, replacing an items is sometimes critical

MutateInResult result = collection.mutateIn("hotel_1501",

    Collections.singletonList(MutateInSpec.replace("foo[1]", newFooObject)));

Reporter: Roi Katz
E-mail: roi.katz@Couchbase.com