Docs: Add max_streams to Promtail configuration doc (#7489)
Docs: Add max_streams to Promtail configuration doc (#7489)

**What this PR does / why we need it**:
Document `max_streams` configuration, introduced in 2.7.

Co-authored-by: Ursula Kallio <7fe6bc33582727447614534a5277d3fb5b1ca290@grafana.com>
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