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docs: update MetricsQL.Subquery section description (#1951)

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* simplify sentences;
* fix typo.

Signed-off-by: hagen1778 <roman@victoriametrics.com>
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@ -918,11 +918,11 @@ See also [implicit query conversions](#implicit-query-conversions).
## Subqueries
MetricsQL supports and extends PromQL subqueries. See [this article](https://valyala.medium.com/prometheus-subqueries-in-victoriametrics-9b1492b720b3) for details. Any nested [rollup functions](#rollup-functions) form a subquery. Nested rollup functions can be implicit thanks to the [implicit query conversions](#implicit-query-conversions). For example, `delta(sum(m))` is implicitly converted to `delta(sum(default_rollup(m[1i]))[1i:1i])`, so it becomes a subquery, since it contains [default_rollup](#default_rollup) nested into [delta](#delta).
MetricsQL supports and extends PromQL subqueries. See [this article](https://valyala.medium.com/prometheus-subqueries-in-victoriametrics-9b1492b720b3) for details. Any [rollup function](#rollup-functions) for something other than [series selector](https://prometheus.io/docs/prometheus/latest/querying/basics/#time-series-selectors) form a subquery. Nested rollup functions can be implicit thanks to the [implicit query conversions](#implicit-query-conversions). For example, `delta(sum(m))` is implicitly converted to `delta(sum(default_rollup(m[1i]))[1i:1i])`, so it becomes a subquery, since it contains [default_rollup](#default_rollup) nested into [delta](#delta).
VictoriaMetrics performs subqueries in the following way:
* It calculates the inner rollup function using the `step` value from the outer rollup function. For example, if `max_over_time(rate(http_requests_total[5m])[1h:30s])` is executed, then the `rate(http_requests_total[5m])` is calculated with the `step` equal to `30s`. The resulting data points are algined by the `step`.
* It calculates the inner rollup function using the `step` value from the outer rollup function. For example, for expression `max_over_time(rate(http_requests_total[5m])[1h:30s])` the inner function `rate(http_requests_total[5m])` is calculated with `step=30s`. The resulting data points are aligned by the `step`.
* It calculates the outer rollup function over the results of the inner rollup function using the `step` value passed by Grafana to [/api/v1/query_range](https://prometheus.io/docs/prometheus/latest/querying/api/#range-queries).