Subquery optimization: Updates docs

[Simran-B]

There is an ongoing effort to improve subquery performance, which seems to introduce new Node types to the execution plan / explain output. The docs probably need an update when this is done.

[Simran-B]

SubqueryStart, SubqueryEnd
https://www.arangodb.com/docs/3.6/aql/execution-and-performance-optimizer.html#list-of-execution-nodes

splice-subqueries
https://www.arangodb.com/docs/3.6/aql/execution-and-performance-optimizer.html#list-of-optimizer-rules