Add clarification about possible values for duration/time fields

[heitortsergent]

Prometheus remote write has the K6_PROMETHEUS_RW_PUSH_INTERVAL variable, which sets the push time interval based on a string value. The example in the docs says the default value is 5s, but doesn't explicitly state what are the available options for that field besides s.

We should review any duration/time fields that follow the same pattern and make sure their description is clear about what are the accepted values.

Chatting with @codebien, we use the ParseDuration which accepts s, m, h, and we also support d for days.