docs: explain counter_server_query_cache_hit and its effect

[anderseknert]

The REST API docs on performance metrics makes no mention of the counter_server_query_cache_hit attributed returned in the response (with the metrics=true query parameter set), nor its effects on the other attributes returned, such as timer_rego_query_compile_ns and timer_rego_query_parse_ns dissapearing from the responses (since they are... well, cached ;))

Since the question of query caching (or even if OPA does decision caching) comes up every now and then, it would be good to have some docs to point to.

[stale]

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