Document that idle_duration requires a nodes range

[r4victor]

The docs promised that idle_duration terminates idle instances of a fixed-size fleet, which can never happen: termination only removes instances above nodes.min, and nodes: N sets min to N.

• Rewrite the "Idle duration" section in concepts/fleets.md to scope termination to instances above nodes.min and switch its example from nodes: 2 to a min: 0 / target: 2 range, so the example actually terminates something.
• Add an admonition explaining that a fleet already at nodes.min keeps its instances, since dstack would immediately provision new ones to maintain the minimum.
• Qualify the same claim in the manage-fleets snippet, which the dev environments, tasks, and services pages include, and in its duplicate in guides/protips.md.
• Extend the idle_duration field description with the fixed-nodes: N consequence, which renders into the fleet reference.

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