Clarify envvar configuration requirement (#141)

* Update configuration.md

Soften the normative requirement for all code to support environment variables to a recommendation. RUM libraries are already documented and specified to not support this configuration method, therefore there exist valid reasons why this part of the specification will not be implementable and should be a recommendation.

* Update with conditional requirement

specification/configuration.md

@@ -2,17 +2,23 @@
 
 **Status**: [Stable](../README.md#versioning-and-status-of-the-specification)
 
-One or more configuration variables MAY be needed to properly configure GDI
-repositories. Configuration of these variables MUST be supported by environment
-variables and MAY be supported by additional methods. GDI repositories MUST adopt
-stable and SHOULD adopt experimental configuration variables in the
-[OpenTelemetry
+One or more configuration variables may be needed to properly configure GDI
+repositories. Components that can be configured with environment variables MUST
+support configuration of these variables using environment variables. Any
+component that cannot be configured with environment variables MUST support
+configuration of these variables using an alternate method and that method MUST
+be defined in a section below (see the [Real User Monitoring Libraries
+section](#real-user-monitoring-libraries) as an example). Any component MAY
+support configuration of these variables by additional methods.
+
+GDI repositories MUST adopt stable and SHOULD adopt experimental configuration
+variables in the [OpenTelemetry
 Specification](https://github.com/open-telemetry/opentelemetry-specification)
 before proposing variables to the GDI specification. If a new configuration
-variable is needed by a GDI repository it MUST be brought to the GDI specification
-as a GitHub issue. The GDI specification maintainers SHOULD consider
-introducing needed configuration variables to the OpenTelemetry repository before
-approving Splunk-specific configuration variables.
+variable is needed by a GDI repository it MUST be brought to the GDI
+specification as a GitHub issue. The GDI specification maintainers SHOULD
+consider introducing needed configuration variables to the OpenTelemetry
+repository before approving Splunk-specific configuration variables.
 
 If a GDI repository requires an immediate configuration variable that is not
 available in the OpenTelemetry specification and not available in the GDI