Document prepared statements behaviour

Document prepared statements optimisation/cost. Some information available in a [blog post](https://hasura.io/blog/fast-graphql-execution-with-query-caching-prepared-statements/). More information on cost of using this feature is also needed.

No file, test, or documentation entry point is identified in the issue. Start by reviewing the linked blog post and the repository's existing documentation for prepared statements; the work is done when the optimization behavior and its usage cost are documented with enough detail for users to understand the tradeoffs.

Written by the indexing model from the issue text.