Document collection behavior of transaction_contention_events

[awwx]

Page: https://www.cockroachlabs.com/docs/v25.3/crdb-internal.html#transaction_contention_events

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Additional details

In cockroachdb/cockroach#149613 (comment), @alyshanjahani-crl described the expected collection behavior of the crdb_internal.transaction_contention_events table.

Without knowing what the collection behavior of the table is supposed to be, the behavior of the table is confusing: it appears random that sometimes a contention event is recorded and sometimes it isn't.

Documenting the collection behavior of the table would avoid this confusion, avoid the appearance of a bug, and explain how the table can be used.

Jira issue: DOC-14823