Clarify that FROM is inclusive and TO is exclusive in range partitioning

[fabiog1901]

On the "Define a range partition on an index" section, it's not documented whether the FROM and TO bounds in PARTITION ... VALUES FROM (...) TO (...) are inclusive or exclusive.

For example:

ALTER TABLE mq PARTITION BY RANGE (bucket) (
    PARTITION mq_0 VALUES FROM (0)   TO (86),
    PARTITION mq_1 VALUES FROM (86)  TO (173),
    PARTITION mq_2 VALUES FROM (173) TO (256)
);

Since adjacent partitions share boundary values (e.g. 86 appears as both the TO of mq_0 and the FROM of mq_1), it's ambiguous from the docs alone which partition a row with bucket = 86 would land in.

To confirm: FROM is inclusive and TO is exclusive (i.e. ranges behave like [FROM, TO)).

Please add a note clarifying this inclusive/exclusive behavior on this page:
https://www.cockroachlabs.com/docs/v26.2/alter-index#define-a-range-partition-on-an-index

Source: this was discussed in Slack here: https://cockroachlabs.slack.com/archives/C04U1BTF8/p1785869947752589?thread_ts=1785869947.752589&cid=C04U1BTF8

Jira issue: DOC-18294