Clarify gRPC insecure option (#2476)

Fixes #2420

## Changes

- Clarify the insecure option for gRPC exporters

Related issues

- open-telemetry/opentelemetry-js#2827

specification/protocol/exporter.md

@@ -12,11 +12,11 @@ The following configuration options MUST be available to configure the OTLP expo
   - Default:  `http://localhost:4318` [1]
   - Env vars: `OTEL_EXPORTER_OTLP_ENDPOINT` `OTEL_EXPORTER_OTLP_TRACES_ENDPOINT` `OTEL_EXPORTER_OTLP_METRICS_ENDPOINT`
 
-- **Endpoint (OTLP/gRPC)**: Target to which the exporter is going to send spans or metrics. The endpoint SHOULD accept any form allowed by the underlying gRPC client implementation. Additionally, the endpoint MUST accept a URL with a scheme of either `http` or `https`. A scheme of `https` indicates a secure connection and takes precedence over the `insecure` configuration setting. If the gRPC client implementation does not support an endpoint with a scheme of `http` or `https` then the endpoint SHOULD be transformed to the most sensible format for that implementation.
+- **Endpoint (OTLP/gRPC)**: Target to which the exporter is going to send spans or metrics. The endpoint SHOULD accept any form allowed by the underlying gRPC client implementation. Additionally, the endpoint MUST accept a URL with a scheme of either `http` or `https`. A scheme of `https` indicates a secure connection and takes precedence over the `insecure` configuration setting. A scheme of `http` indicates an insecure connection and takes precedence over the `insecure` configuration setting. If the gRPC client implementation does not support an endpoint with a scheme of `http` or `https` then the endpoint SHOULD be transformed to the most sensible format for that implementation.
   - Default: `http://localhost:4317` [1]
   - Env vars: `OTEL_EXPORTER_OTLP_ENDPOINT` `OTEL_EXPORTER_OTLP_TRACES_ENDPOINT` `OTEL_EXPORTER_OTLP_METRICS_ENDPOINT`
 
-- **Insecure**: Whether to enable client transport security for the exporter's gRPC connection. This option only applies to OTLP/gRPC - OTLP/HTTP always uses the scheme provided for the `endpoint`. Implementations MAY choose to not implement the `insecure` option if it is not required or supported by the underlying gRPC client implementation.
+- **Insecure**: Whether to enable client transport security for the exporter's gRPC connection. This option only applies to OTLP/gRPC when an endpoint is provided without the `http` or `https` scheme - OTLP/HTTP always uses the scheme provided for the `endpoint`. Implementations MAY choose to not implement the `insecure` option if it is not required or supported by the underlying gRPC client implementation.
   - Default: `false`
   - Env vars: `OTEL_EXPORTER_OTLP_INSECURE` `OTEL_EXPORTER_OTLP_TRACES_INSECURE` `OTEL_EXPORTER_OTLP_METRICS_INSECURE` `OTEL_EXPORTER_OTLP_SPAN_INSECURE` `OTEL_EXPORTER_OTLP_METRIC_INSECURE` [2]