fix comments for unit support

Signed-off-by: Arianna Vespri <arianna.vespri@yahoo.it>

expfmt/openmetrics_create.go

@@ -47,12 +47,15 @@ import (
 // Prometheus to OpenMetrics or vice versa:
 //
 //   - Counters are expected to have the `_total` suffix in their metric name. In
-//     the output, the suffix will be truncated from the `# TYPE` and `# HELP`
-//     line. A counter with a missing `_total` suffix is not an error. However,
+//     the output, the suffix will be truncated from the `# TYPE`, `# HELP` and `# UNIT`
+//     lines. A counter with a missing `_total` suffix is not an error. However,
 //     its type will be set to `unknown` in that case to avoid invalid OpenMetrics
 //     output.
 //
-//   - No support for the following (optional) features: `# UNIT` line, `_created`
+//   - The `# UNIT` line is optional, but if populated, the unit has to be present in the metric
+//     name as its suffix (see https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#unit)
+//
+//   - No support for the following (optional) features: `_created`
 //     line, info type, stateset type, gaugehistogram type.
 //
 //   - The size of exemplar labels is not checked (i.e. it's possible to create