[api] Fix some doc comments

[david-crespo]

Closes #9359, closes #4942, closes #7649, and redoes #9871.

• silo_user_list and silo_user_view said "built-in (system) users", but they are for regular silo users ([api] Wrong description on silo_user_list and silo_user_view #9359)
• Clarify that the audit log start_time and end_time params filter on time_completed (redo of docs: clarify audit log filtering behavior. #9871)
• Add missing doc comment on SiloCreate.discoverable and reword the matching one on the Silo view (Missing description under discoverable in SiloCreate #4942)
• Reword AffinityPolicy variant docs to avoid the ambiguous "affinity request": constraints apply when an instance starts, and with fail the start request errors and the instance remains stopped (Clarify "affinity request" in policy doc comment #7649)

Also say "network interface" instead of "OPTE" in the jumbo frames doc comments — noticed in the discussion on oxidecomputer/oxide.rs#1525, where the OPTE wording had been copied into the CLI. "Primary network interface" is accurate: the jumbo MTU is applied only to the primary NIC's OPTE port, and only when both the fleet flag and the instance bit are set.

omicron/nexus/src/app/instance.rs

Lines 2923 to 2935 in bc1e632

	/// Effective MTU for an instance's primary OPTE port. Jumbo frames are only
	/// applied when both the fleet-wide opt-in is enabled and the per-instance bit
	/// is set; otherwise this returns `None` and OPTE applies the default MTU.
	fn primary_nic_mtu_for_jumbo_frames(
	instance_jumbo_frames_enabled: bool,
	fleet_jumbo_frames_opt_in_enabled: bool,
	) -> Option<u32> {
	if instance_jumbo_frames_enabled && fleet_jumbo_frames_opt_in_enabled {
	Some(omicron_common::address::EXTERNAL_JUMBO_FRAMES_MTU)
	} else {
	None
	}
	}