Document collection behavior of transaction_contention_events

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

## What is the reason for your feedback?

[X] Missing the information I need

[ ] Too complicated

[ ] Out of date

[ ] Something is broken

[ ] Other

## Additional details

In https://github.com/cockroachdb/cockroach/issues/149613#issuecomment-3225419219, @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