monitoring: clarify selected_regions cost and set 3-checker minimum in uptime check examples

[darren-manukyan]

Clarify documentation on selected_regions for google_monitoring_uptime_check_config and set selected_regions = ["USA"] in canonical documentation examples.

• Explains in the schema description that leaving selected_regions unset defaults to all 6 available regional checkers (USA_OREGON, USA_IOWA, USA_VIRGINIA, EUROPE, SOUTH_AMERICA, ASIA_PACIFIC), which incurs 6 probe executions per period.
• Documents the 3-location API minimum and points out that specifying ["USA"] covers 3 locations.
• Sets selected_regions = ["USA"] in uptime_check_config_http and uptime_check_config_https sample templates so users copy-pasting documentation examples default to the lowest-cost valid configuration.

Fixes hashicorp/terraform-provider-google#28764

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