Adding documentation to default constructor.

[conniey]

Description

Fixes javadoc warning below. The default constructor is Deprecated.

[WARNING] Javadoc Warnings
[WARNING] /mnt/vss/_work/1/s/sdk/schemaregistry/azure-data-schemaregistry/src/main/java/com/azure/data/schemaregistry/models/SchemaFormat.java:13: warning: use of default constructor, which does not provide a comment
[WARNING] public final class SchemaFormat extends ExpandableStringEnum<SchemaFormat> {
}

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• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
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[azure-sdk]

API change check

APIView has identified API level changes in this PR and created following API reviews.

azure-data-schemaregistry