Clarify declarative config default requirements (#4780)

When the declarative config `create` spec was written, declarative
config was dependent on the spec as its source of truth for default
values for the various properties.

We now directly record expected [`defaultBehavior` and
`nullBehavior`](https://github.com/open-telemetry/opentelemetry-configuration/blob/main/CONTRIBUTING.md#json-schema-source-and-output)
semantics in `opentelemetry-configuration`.

Updating the spec to tie things back together. 

/cc @open-telemetry/configuration-approvers

Author: jack-berg

specification/configuration/sdk.md

@@ -239,7 +239,9 @@ Parse SHOULD return an error if:
 
 * The `file` doesn't exist or is invalid
 * The parsed `file` content does not conform to
-  the [configuration model](data-model.md) schema.
+  the [configuration model](data-model.md) schema. Note that this includes
+  enforcing all constraints encoded into the schema (e.g. required properties
+  are present, that properties adhere to specified types, etc.).
 
 #### Create
 
@@ -260,15 +262,31 @@ Interpret configuration model and return SDK components.
 The multiple responses MAY be returned using a tuple, or some other data
 structure encapsulating the components.
 
-If a property has a default value defined (i.e. is _not_ required) and is
-missing or present but null, Create MUST ensure the SDK component is configured
-with the default value. If a property is required and is missing or present but
-null, Create SHOULD return an error. For example, if configuring
-the [span batching processor](../trace/sdk.md#batching-processor) and
-the `scheduleDelayMillis` property is missing or present but null, the component
-is configured with the default value of `5000`. However, if the `exporter`
-property is missing or present but null, Create fails fast since there is no
-default value for `exporter`.
+Create requirements around default and null behavior are described below. Note that
+[`defaultBehavior` and `nullBehavior`](https://github.com/open-telemetry/opentelemetry-configuration/blob/main/CONTRIBUTING.md#json-schema-source-and-output)
+are defined in the configuration data model.
+
+* If a property is present and the value is null, Create MUST use the
+  `nullBehavior`, or `defaultBehavior` if `nullBehavior` is not set.
+* If a property is required, and not present, Create MUST return an error.
+
+A few examples to illustrate:
+
+* If configuring [`BatchSpanProcessor`](https://github.com/open-telemetry/opentelemetry-configuration/blob/main/schema-docs.md#batchspanprocessor-)
+  and `schedule_delay` is not present or present but null, the component is
+  configured according to the `defaultBehavior` of `5000`.
+* If configuring [`SpanExporter`](https://github.com/open-telemetry/opentelemetry-configuration/blob/main/schema-docs.md#spanexporter)
+  and `console` is present and null, the component is configured with a
+  `console` exporter with default configuration since `console` is nullable.
+
+The [configuration model](data-model.md) uses the JSON schema
+[`description`](https://json-schema.org/understanding-json-schema/reference/annotations)
+annotation to capture property semantics which cannot be encoded using standard
+JSON schema keywords. Create SHOULD return an error if it encounters a value
+which is invalid according to the property `description`. For example, if
+configuring [`HttpTls`](https://github.com/open-telemetry/opentelemetry-configuration/blob/main/schema-docs.md#httptls-)
+and `ca_file` is not an absolute file path as defined in the property
+description, return an error.
 
 When encountering a reference to
 a [SDK extension component](#sdk-extension-components) which is not built in to